Recent Major Advances and Opportunities in AMO Science and Applications to the Needs of Society
The intellectual excitement and rate of progress in the field of AMO science are at an all-time high. The discovery and development of the laser and of other innovative techniques make this a time of unparalleled scientific opportunity, and these discoveries have led to scientific and technological advances that once could only be dreamed of. AMO science has enabled major technological advances in manufacturing, materials, communications, space, defense, energy, the environment, health, and transportation that have had a major impact on the nation's economic productivity, competitive position, security, and technological infrastructure and on the general well-being of its people. This chapter provides examples that highlight recent advances in AMO science and its applications and that illustrate the promise of the field.
Some of the examples included below have been discussed in a recent National Research Council (NRC) report, Research Briefing on Selected Opportunities in Atomic, Molecular, and Optical Sciences (National Academy Press, Washington, D.C., 1991). More detailed discussions of many of these topics can be found in the report Future Research Opportunities in Atomic, Molecular, and Optical Physics (PUB-5305, Lawrence Berkeley Laboratory (LBL), Berkeley, California, 1991), sponsored by the Department of Energy (DOE). Here, as in the earlier reports, it is only possible to touch on a few of the accomplishments and opportunities because their number is so large.
THE NATION'S SCIENTIFIC KNOWLEDGE BASE
New and revolutionary discoveries of a fundamental nature continue to be made in AMO science, and, as demonstrated by the citation analysis presented in
Appendix C, the United States is a world leader in this area. To identify scientific forefronts, technological opportunities, and windows of future opportunity, broad input was sought from the AMO science community through letters to individual scientists, open sessions at professional society meetings, and other forums. That input identified the areas presented below as those meriting special discussion. Clearly, not all areas of significant progress can be included in such a brief presentation. Nevertheless, this summary will provide a glimpse of some of the scientific frontiers in AMO science.
Recent Discoveries and Future Opportunities in AMO Science
Fundamental Laws and Symmetries
One unique aspect of AMO physics, distinguishing it in all of science, is the capacity to make measurements with extraordinarily high precision. In suitably chosen systems, such precision measurements can probe physics far beyond the confines of what is customarily considered AMO science. For example, precision AMO measurements are testing our basic concepts of space and time, revealing new details about nuclear structure, probing the existence and properties of elementary particles, and exploring our fundamental understanding of the forces of nature. In this way, AMO science provides the unusual opportunity to explore the frontiers of physics without leaving the proverbial "tabletop." A common theme in much of this work is that one tests accepted theories at increasingly higher levels of precision until, at some point, a discrepancy is observed, which leads to important new insights.
One example is the testing of the ideas of space and time that are embodied in the theory of special relativity. The optical experiments of Michelson and Morley and of Kennedy and Thorndike provided important early tests of the isotropy of space and the speed of light. Recently, laser versions of these experiments have tested the isotropy hypothesis at a precision many orders of magnitude higher. Similarly, recent laser spectroscopy experiments have provided dramatically improved precision for the confirmation of the time dilation formula of special relativity.
The area of precision measurements that has made perhaps the largest contribution to the basic understanding of physics is the detailed examination of atomic structure. Historically, precision measurements of atomic and molecular spectra laid much of the groundwork for the development of quantum mechanics, and the high precision of these data provided an exceptionally rigorous testing ground for new theories as they were put forth. The microwave technology developed in World War II led to the precise measurement of the Lamb shift in hydrogen, which stimulated the modern development of quantum field theory. The rather radical concepts of renormalization and vacuum fluctuations gained quick acceptance because of the remarkable agreement between theoretical predictions
and experiment. The development of quantum electrodynamics has advanced to the point that calculated and measured values of the magnetic moment of the electron agree to 1 part in 109.
This same spirit underlies the ongoing efforts to find and study new, non-Coulomb forces through their effects on atomic structure. Particular attention has been given to forces that violate time (T) and/or parity (mirror) (P) reversal symmetries. At present the only evidence for a time-reversal-violating force is in the neutral kaon system, and there is much speculation, but no solid arguments, as to its origin and relationship with other forces. An electric dipole moment (EDM) of a fundamental particle could only exist if there were a T-violating force, so there has been a long history of experiments that have searched for EDMs of neutrons, electrons, atoms, and nuclei. Over the years, these measurements have improved enormously and now have achieved extraordinary levels of sensitivity. For example, the present limit on the EDM of an atom is equivalent to a displacement of the positive and negative charges of about 10-26 cm. The fact that EDM searches have yet to yield a nonzero result places severe constraints on theoretical models. Indeed, the standard model is one of the few that have been put forth that can simultaneously explain the presence of T-violation in kaons and its absence at the level currently set by experiments in atomic systems. It is generally believed, however, that the standard model is only a part of a larger scheme, such as supersymmetry. EDM searches are one powerful means to test many of the ideas put forth in these larger schemes.
The study of parity nonconserving forces in atoms is, in a historical sense, positioned between the Lamb shift and the search for the electric dipole moments. The Lamb shift was at one time a radical new discovery but is now accepted without question, whereas the search for EDMs is a quest (as yet unfulfilled) to observe a new phenomenon. Throughout this century, there have been numerous experiments that set limits on parity nonconservation (PNC) in atoms, but it has only been in the last 10 to 15 years that experimental precision reached the necessary level to detect this effect. Although atomic PNC is at the extremely small level of only 1 part in 1011 mixing of atomic states, it is now possible to measure the size of this mixing, for example in a cesium atom, to 2% accuracy. PNC in atoms is understood to be a manifestation of the neutral weak interaction that was predicted as a result of efforts to unify the weak and electromagnetic forces. The study of the neutral weak force remains one of the best methods of testing the standard model of electroweak unification and probing the many unexplained features of this model. By themselves, the experimental atomic PNC data are not sufficient to probe the nature of the neutral weak force. In order to interpret the data, it is also necessary to precisely calculate the structure of many-electron atoms such as cesium. In the past few years, dramatic advances in computational techniques have made this possible. This "marriage" of theory and measurement of atomic PNC now provides one the most precise tests of the standard model and complements the many high-energy tests of the model,
because only the atomic experiments are sensitive to the values of two of the four basic electron-quark neutral current coupling constants. A substantial number of new theories have been put forth to avoid the problems of the standard model, and their primary observable effects involve these two coupling constants. Because of this, the atomic PNC results set the most stringent constraints on much of this so-called ''new physics." As these experiments and the atomic structure calculations are improved, they will further probe the nature of the electroweak unification. Only a few other experiments have comparable potential to address basic questions in elementary particle physics, and these are generally being conducted within the high-energy physics community.
Precision AMO measurements of the sort discussed here drive, and are driven by, the technology of the entire AMO field. For example, the progress in the search for EDMs has directly reflected improvements in radio frequency resonance and, more recently, in laser spectroscopic techniques. Work is now under way to incorporate laser cooling techniques into these searches, which promises to lead to dramatic improvements. Efforts to improve special relativity tests and atomic PNC experiments have led to developments in laser stabilization and optics that are now spreading throughout the field of AMO science and, in particular, have had a substantial impact on techniques for laser cooling and trapping of atoms. This mutually beneficial relationship will continue to advance the "tabletop frontier," as well as further drive the technology that has made AMO science such a vital field.
Cavity Electrodynamics and Micromasers
Cavity quantum electrodynamics (QED) deals with the modification of free-space atomic radiation processes by cavities and other structures. Although it has been nearly 50 years since such effects were first considered, it is only in the past decade that experimental techniques, especially the use of Rydberg atoms and superconducting cavity walls, have become available to study such effects with single atoms. These techniques have allowed the observation of the exchange of energy between an atom and a single mode of the electromagnetic field in a cavity, which has been successfully modeled theoretically. When this treatment is extended to allow for the coupling of the atom to an arbitrary number of modes, the sinusoidal exchange of energy is generally replaced by an effectively irreversible transfer of energy from an excited atom to the field. The effect of the cavity is then to modify the atom's spontaneous emission in a way that depends on such things as the position of the atom within the cavity and the reflectivities of the cavity walls. Spontaneous emission can be inhibited if there are no allowed cavity modes at the emission frequency. Various experiments have verified the predicted modifications of radiative lifetimes by cavities.
Recent developments have also opened the possibility of experimental studies of the transition between small and large systems. Such studies promise to
shed light on the few-body problem and classical/quantum correspondence. In so-called micromasers a low-density beam of Rydberg atoms is injected into a single-mode microwave cavity at such a low rate that at most one atom at a time is in the cavity. As such, micromasers are dynamically driven systems: both the cavity mode and the atoms are dynamical systems, so that the cavity mode in particular is an open system that can evolve from mixed states to pure states. Since good photon detectors in the microwave range are not available, one studies instead the state of the Rydberg atoms as they exit the cavity. The atoms play the dual role of pump and detector. This measurement scheme makes the micromaser a particularly attractive test system to investigate a number of aspects of quantum measurement theory. In addition to the issues in quantum dynamics and measurement theory, micromasers are theoretically attractive because the amplifying medium is relatively simple and an accurate quantum treatment is possible. Quantum fluctuations play an important role in these systems, since the mean photon number in the cavity is extremely low.
Laser action is possible in the micromaser because the field amplification by a single atom is sufficient to overcome the tiny loss of the cavity. It is possible using micro-optical, non-superconducting cavities to achieve laser action in many-atom systems with nearly arbitrarily low pumping levels, and without any distinct pumping threshold. This laser action occurs because in a micro-optical cavity, where one dimension is of the order of half a wavelength long, photons are emitted into a single mode, without spontaneous emission into all nonlasing modes, as in a conventional laser. Increased pumping then results in a gradual transition from predominantly spontaneous to predominantly stimulated emission, without a sharp threshold. Thresholdless lasing has been demonstrated with microcavities containing dye solutions and should be possible also with semiconductors. The ultralow power consumption of such devices makes them interesting for various applications. Microcavity lasers also offer response rates exceeding 100 gigabits (100 billion bits) per second, which cannot be realized with more conventional lasers.
Efficiencies of such important semiconductor devices as laser diodes and solar cells are strongly determined by electron-hole radiative recombination rates. Radiative decay rates can be suppressed in geometries having no allowed electromagnetic modes at the radiative wavelengths, as already noted. Inhibition can also be realized by forming structures in such a way as to produce photonic band gaps, that is, regions in a transmission versus wavelength curve where transmission is forbidden, analogous to forbidden energy bands of electrons in crystals. Recently, a cubic lattice with a photonic band gap having a width of about 20% of the central (optical) frequency has been fabricated. Such structures, which are an extension of the ideas of cavity QED, offer the possibility of dramatically improving the efficiencies of various electronic devices.
Although photonic band gap structures are currently in a basic research stage, it is not difficult to imagine possible practical applications. For instance,
they might be used in lasers to inhibit radiative decay into nonlasing modes. This could be commercially important because it might lead to a substantial reduction in pumping requirements of diode lasers and contribute to the development of thresholdless lasers.
Highly Perturbed Atoms in Intense Laser and Microwave Fields
The interaction between laser light and atoms has been an active area of experimental and theoretical research since the discovery of the laser. By the early 1980s, these interactions were thought to be reasonably well understood, based on well-characterized perturbation theory. At about that time, however, improvements in laser technology led to intensities approaching 1013 watts per square centimeter (W cm-2), where the ponderomotive, or "quiver," energy of the electron in the field becomes comparable to the photon energy. Intensities a few orders of magnitude higher are now possible, providing laser fields comparable to the strength of the electric fields that hold an atom together. At these intensities, where the effect of the laser light on the atom is not a small perturbation, entirely unexpected phenomena began to be observed. Experiments measuring the energy distribution of photoejected electrons showed large peaks corresponding to the absorption of many more photons than were necessary to ionize the atom. Such processes, now known as above-threshold ionization (ATI), had previously been thought to be of marginal importance but were found to dominate the spectrum at high laser intensities. Other unanticipated phenomena were also discovered, such as inexplicably high probabilities for multiple ionization of atoms by strong laser fields. Unexpected results had been observed earlier in studies of multiphoton ionization of atoms in high-lying Rydberg states by strong microwave fields.
It is now clear that these various experiments were signaling the entry of atomic physics into the realm of strongly coupled systems, where perturbation theories no longer can be depended on to provide descriptions of atomic behavior. Entry into this new realm pushed atomic theory in two different directions. The first was away from simpler perturbative approaches and into extensive computer analyses of the detailed quantum mechanics of these problems. Because these become extremely complex and difficult, a complete analysis is not yet possible even with present computational capabilities. However, calculations of appropriate model problems have now qualitatively reproduced most of the experimentally observed features. One intriguing prediction of these calculations is that with increasing field intensity in the strong field regime atoms can actually be stabilized against ionization. This counterintuitive prediction suggests that ionization probabilities are not, in all cases, monotonically increasing functions of field intensity.
The second theoretical approach to describing the behavior of atoms in intense laser fields has been through the broad field of nonlinear dynamics and
chaos and, more specifically, through the investigation of quantum dynamics in systems with chaotic classical limits. These investigations, often referred to as "quantum chaos," mark a paradigm shift in atomic physics and include the study of statistics of energy-level spacings, the phenomenon of "dynamical localization" representing the suppression of diffusive behavior seen in the classical limit, and "scarring," the peaking of eigenstates of the time-evolution operator or quasi-energy states on unstable classical invariant structures. Because studies of nonlinear dynamics and chaos tend to emphasize universal aspects of the phenomena, this suggests that atomic physics might be an important testing ground for the development of new ideas having application throughout the physical world.
Microwave experiments involving Rydberg states of hydrogen have provided evidence for scarring and dynamical localization. These mechanisms provide the most complete and compact explanation of the experimental observations, while a more conventional interpretation is either exceedingly cumbersome or not possible. Recent efforts to extend the same approach to problems of ATI have led to the suggestion that scarring is relevant to the interaction of ground state atoms with intense lasers. The universality of the ideas of chaos and nonlinear dynamics links these studies to recent investigations of such seemingly unrelated areas of atomic physics as the spectrum of the diamagnetic hydrogen atom, the doubly excited spectrum of the helium atom, Rydberg charge transfer, and the motion of charged particles in traps. These well-characterized and controlled studies in atomic physics can then serve as paradigms for higher-dimensional problems in other areas, including atomic or molecular collisions that are not weak and cannot be considered slow or fast, driven quantum wells, and other mesoscopic solid-state systems.
Transient States of Atomic Systems and Collision Dynamics
The key to understanding a vast array of complex atomic collision phenomena involving the transfer of energy, angular momentum, and charge is, in many cases, the accurate description of the transient intermediate states of the collision complex. Improved experimental capabilities are making possible far more complete analyses of such states than have been heretofore possible, thereby permitting stringent tests of theory. In addition, a host of novel physical processes are being studied experimentally for which a theoretical understanding is only beginning to be developed. The scope of such processes encompasses all the various interactions between photons, electrons, positive and negative ions, neutral atoms, and even antiparticles.
Dynamics of Three-Body Systems. While systems that comprise two interacting particles may be described analytically, systems having three or more interacting particles can be described only approximately. Three-body systems are thus the
prototypes of many-body systems. Experimental and theoretical progress in their description is a key to much of the physics of the everyday world. Indeed, the physics of many complex processes is governed by the interactions of three particles (one or more of which is often a composite particle). Important progress has recently been made in several areas.
High-resolution experimental measurements of the photodetachment cross section for the negative hydrogen ion have uncovered an extremely rich spectrum of doubly excited states, and this has been accompanied by commensurate theoretical advances. Experimental measurements of photo double ionization of atoms have revealed an intriguing empirical relationship between this process and electron impact ionization of the corresponding singly charged ions. The measurements are so precise that they have also tested theoretical understanding of the threshold laws for three-body breakup and, indeed, have led theory to predict alternative modes for three-body breakup applicable in different energy regions above threshold. Finally, the experimental measurements at high photon energies are providing severe tests of theoretical estimates of the ratio of double to single photoionization at asymptotically high photon energies.
Resonances in low-energy electron-atom and electron-molecule collisions provide detailed information on transient states of negative ions. Many such states are doubly excited, and electron-correlation effects are of paramount importance in their theoretical description. Several years ago, experimental data on electron-cesium scattering were used to make a semiempirical prediction of a stable 3P state of the negative cesium ion. The existence of this state has more recently been predicted by ab initio theoretical calculations and confirmed by direct experimental observation.
Collisions at Ultralow Temperatures. Recent advances in laser cooling and manipulation of alkali, alkaline earth, and rare-gas metastable atoms have opened up many new opportunities in atomic collision studies. In particular, these advances now permit the study of inelastic energy transfer and associative and Penning ionization reactions at temperatures below 1 millikelvin (mK). The study of such collisions is important for both practical and fundamental reasons. Collisions limit achievable trap densities and give rise to difficulties in intended applications, such as preventing the realization of Bose-Einstein condensation of cold trapped atoms or causing pressure shifts in high-precision atomic clocks. Low-temperature collisions also display a number of unusual characteristics. Two distinctly different classes of collisions can occur. Collisions of ground or metastable states can be described by conventional scattering theory. In the near-threshold regime, only s-wave collisions have nonvanishing inelastic rates, and the rate coefficients are sensitive to the spin statistics of the colliding atoms and are subject to manipulation by external magnetic fields. In contrast, if laser radiation tuned near the cooling transition is present, collisions involving excited states will occur. These collisions are controlled by an extremely long range
resonant dipolar interaction and enable a new kind of molecular spectroscopy, which probes the long-range potentials of the excited molecule formed from the two atoms and can be used to study molecular bound states having turning points of up to many hundreds of angstroms. Collision rates can be manipulated optically by varying the detuning and intensity of the radiation. If the detuning from resonance is only a few natural line widths, the atoms can be excited only when they are far apart, and they undergo optical pumping and spontaneous decay as they interact during their slow approach to one another. Conventional scattering theory no longer suffices, and new theoretical methods must be developed to account for the fluctuations and dissipation during the long-range part of the collision. Numerous experimental and theoretical studies of various aspects of the physics of ultracold collisions are now beginning, and this subject promises to have an exciting future.
Highly Charged Ions. Highly charged ions dominate hot plasmas such as those encountered in nuclear fusion reactors, X-ray laser research, and stars. Accurate spectroscopic and collision data are required to model the behavior of such plasmas. The technology for making and handling highly charged ions has advanced dramatically during the past few years, opening a whole new class of phenomena for study. Highly charged ions provide a critical test bed for our fundamental understanding of atomic structure and interactions. The enhanced long-range Coulomb forces between highly charged ions and other charged particles give rise to large cross sections for some processes. The electronic potential energy carried by the ion can overshadow kinetic effects in slow collisions. For highly charged ions, the reduced nuclear screening increases the binding energy of the outer valence electrons. Thus, processes involving inner-shell electrons often dominate as the ionic charge increases, for example, in electron-impact ionization. Electron-electron correlation effects may also be enhanced and elucidated in interactions of highly charged ions, for example, in multiple electron capture.
New Insights into Molecular Dynamics
Recent advances in laser technology have enabled scientists to examine phenomena with femtosecond time resolution. This capability has triggered many developments in AMO science. It is now possible, for example, to observe atoms moving in response to chemical forces, to monitor the flow of energy out of individual chemical bonds, and to contemplate optical control of the outcome of particular reactions (Figure 2.1). Complementary to this experimental understanding of molecular motion are numerous conceptual theoretical breakthroughs permitting, for example, the analysis of static spectra using time-dependent wave packets or the observation of flux propagation through curve crossing regions.
The ability to excite and probe molecules on very fast time scales has greatly enhanced the understanding of energy transfer and relaxation in solids and liquids and of the dynamics of solvation. This is a particularly valuable endeavor because much of the important industrial chemistry occurs in solution or on catalysts. For example, these experiments have begun probing the key steps in solvation and reaction by monitoring the fate of an excited molecule in solution or as individual solvent molecules are added to clusters. Ultrafast techniques have also opened new possibilities for studying interfaces. In some of the first experiments, the energy flow out of surface-adsorbed molecules has been observed directly. These approaches provide new details about charge transfer in molecules adsorbed on electrode surfaces, and nonlinear spectroscopy using the high fields created by ultrashort pulse lasers permits entirely new measurements that reveal the nature of molecules adsorbed on surfaces.
Efforts to understand atomic and molecular interactions have produced fundamental new insights into reactive and nonreactive events. A key new direction involves the complete solution of the "vectorial" nature of collision dynamics problems, requiring elaborate angular momentum analyses and ingenious polarization and imaging technologies. Another is "coherent" control dynamics using preparation of nonstationary states. Many of the recent experimental insights come from the application of lasers to quantum state probing, the development of which has involved atomic, molecular, and optical sciences together. Computer technology has vastly expanded the scope of the accompanying theory, enabling the calculation of potential energy surfaces and dynamics through quantum and classical methods.
By applying lasers to a variety of elementary processes, researchers are exploring atomic and molecular interactions in unprecedented detail. In these experiments, atoms or molecules are prepared in selected initial states, even including selected orientations or alignments in space, and the states are analyzed after reaction or inelastic scattering. This preparation and analysis extends to the control of chemical reactions in a few prototypical cases. The measured orientation dependences provide the underlying basis for an understanding of reaction stereochemistry. One approach to controlling reactions is preparation of eigenstates that have special reactivity, and the other is preparation of nonstationary (coherent) states that allow laser control of the chemistry. In the future, it is envisioned that individual molecular bonds could be broken or formed selectively by the injection of modulated pulses of light with specific frequencies.
Rapid advances in theory, driven by both computational technology and new ideas, are being made. The implementation of time-dependent quantum mechanical methods for practical calculations is one example. The evolution of a quantal wave packet, properly analyzed, provides in one calculation the answers to numerous questions concerning the absorption spectrum and the time evolution on dissociative pathways. Other methods enable very large scale calculations of the structure of complex molecules and of the potential energy surfaces
on which reactions occur. Quantal flux propagation promises to unveil the motion through curve crossings unfettered by the problems of visualizing the interferences between ingoing and outgoing quantum mechanical waves.
Quantum-state-resolved studies of atomic and molecular interactions have opened the door to exciting new possibilities. The close connection between theory and experiment will yield significant advances, with each area stimulating the other. The machinery for these fundamental studies is becoming sophisticated enough to allow modeling of practical systems, some of which urgently require a thorough understanding. Better state and time resolution will incisively test theoretical models and validate their application to ever more complex systems, many with important practical applications and impact.
Clusters are chemical aggregates containing specific numbers of atoms and molecules. Understanding the physics of molecular and atomic clustered species provides the fundamental link between the properties of isolated species and condensed matter. Indeed, the unusual characteristics of clustered species often suggest their role as an additional state of matter, with interesting consequences for catalysis, atmospheric chemistry, and materials. Cluster science uses the tools of AMO research to explore the appearance, with increasing size, of bulk physical and chemical properties. It provides new insights into solid-state physics and has resulted in the discovery of entire new classes of large molecules, as related, for example, in the "buckyball" story in Chapter 1.
In the simple limit, two molecules may be joined by weak van der Waals forces in a low-temperature jet expansion. The study of these weak forces and the often unexpected geometries that ensue is one of the most productive areas of molecular spectroscopy today. The results demonstrate the nature of intermolecular forces in unprecedented detail and have broad implications even for the hydrogen bonding of large biological molecules. Methods for the creation of ever-expanding classes of chemical clusters have evolved using laser ablation sources, jets, and clever chemical kinetics.
It is now possible to study a solvated electron in size-selected clusters and to manufacture metal-carbon clusters of preferential sizes for use as novel reagents. The caging effect of two molecules surrounded by a solvent shell has been studied in detail with ultrafast laser dynamics. Aerosol and ice clusters have been discovered to be important in the stratospheric ozone reduction problem, challenging scientists to devise new methods to study heterogeneous kinetic processes.
Clusters of metals and semiconductors have properties that differ from either the small-molecule or solid-state limits. In simple alkalis such as sodium, collective plasmon optical resonances have been observed in clusters of just a few atoms. In contrast, the bulk ionization potential is approached only in very
large clusters; thus there are both quantum and classical electrostatic size-dependent terms. Highly symmetrical physical structures have been inferred for large metallic clusters from regular patterns of gas phase reaction and chemisorption. In addition, mobility studies show a clear separation of clusters that are linear, spherical, or ring shaped. In simple free-electron metals, the appearance of closed shells of electrons leads to "magic numbers" in stability. Ten-atom clusters of magnetic elements have magnetic properties stronger and functionally different from those of the bulk material—the approach to bulk magnetism is only just beginning to be understood. The many unusual effects of clusters have stimulated substantial theoretical efforts, resulting in large-scale molecular dynamical simulations and the demand for more powerful computational tools.
Semiconductor crystallites, nanocrystals, or quantum dots 2 to 5 nanometers (nm; 10-9 meters) in diameter have band gaps that increase with decreasing size, due to a large electronic quantum size effect. Their optical and electrical properties can be "tuned" by size. Macroscopic amounts of nanocrystals with narrow size distributions and surfaces stabilized by chemical bonding to organic molecules have been produced. Smaller semiconductor clusters (tens of atoms) have structures and electronic properties that are only poorly understood at present. The surfaces of these small clusters are less reactive than the surfaces of the bulk materials. The bulk "unit cell" appears to develop quite slowly with increasing size. With the rapidly advancing understanding of cluster species, the potential to use them in electronics and other devices remains a highly attractive goal.
Physics of Nonlinear Optics
The study of nonlinear phenomena in optics originated with the development of the laser and has resulted in a number of exciting discoveries. One nonlinear phenomenon of great interest is the soliton, a wave that propagates without distortion. Soliton pulses ranging from several hundreds of femtoseconds to a few picoseconds have been generated in optical fibers. The distortion-less propagation of such pulses makes them of obvious interest for optical communications, because they can, in theory, transmit digital data at higher rates and over longer distances than is currently possible. Solitons in fibers have been realized using amplification and nonlinearity associated with stimulated Raman scattering. Solitons have also been generated in erbium-doped fiber lasers combined with undoped fibers. So-called dark solitons—short dips in an otherwise continuous beam of light—are of interest for optical switching.
Other nonlinear methods are being studied to generate coherent radiation. Lasers without inversion, that is, without the need for an auxiliary pumping process to produce a population inversion, have been investigated in the past with Raman interactions and resonant frequency conversion involving only two atomic states. Recently, there has been a resurgence of interest in this area with the development of concepts involving multilevel near-resonant systems. Closely
related is the use of intense laser fields to reduce absorption in resonantly enhanced nonlinear optical frequency conversion. it is reasonable to expect that novel methods for the generation of coherent radiation will continue to be devised in the future.
Nonlinear optical processes have also been employed to produce states of light that display distinctly quantum mechanical properties. In the quantum theory of a mode of the electromagnetic field, two in-quadrature components are defined that play the role of canonical variables. The uncertainty principle sets a limit on the accuracy with which both variables can be simultaneously measured, but it is of course possible to measure one with arbitrary accuracy at the expense of admitting large fluctuations in the other. Thus it is possible to produce ''squeezed" states of the field in which one of these components has very small quantum fluctuations. This capability opens the possibility of encoding and decoding information using that component that is relatively noise-free. Squeezed states can be produced in a variety of nonlinear optical processes. Experiments with atomic beams and collections of two-level atoms in cavities, as well as three-wave mixing processes, have demonstrated noise reductions substantially below shot-noise levels. Squeezed states are of particular interest for the detection of gravitational waves, where the signal is so weak that it would ordinarily be masked by quantum noise. Applications are anticipated in many situations in which it is important to enhance signal-to-noise ratios, and various spectroscopic applications have also been proposed.
Experiments measuring polarization correlations in the two photons produced in a cascade atomic radiative decay have tested some of the most basic aspects of quantum theory vis-à-vis "realistic" local hidden variable theories. These experiments have upheld the so-called Bell inequalities predicted by quantum theory. However, there are still possible loopholes that prevent one from ruling out "realistic" theories with absolute certainty, and new experimental tests of quantum theory have been devised involving nonlinear optics. In these experiments a quasi-monochromatic beam of light is split by a nonlinear medium into two beams, each with a frequency half that of the original beam. Correlations in the positions at which the generated photons are detected, with fixed polarizations, replace the correlations in the polarizations of the emitted photons in the earlier, atomic cascade experiments. These correlations are also subject to Bell-type inequalities. In particular, quantum theory predicts a nonlocal type of interference in spatially separated photon channels, and these effects have been observed.
Progress continues to be made in the understanding of optical instabilities and chaos in lasers and nonlinear optical devices. During the past decade, it has been well established that chaos is by no means an unusual mode of behavior in such systems. Indeed, experiments in this area were among the first to confirm various "universal" routes to chaos predicted theoretically. The focus more recently has been on spatiotemporal chaos associated with variations in the field
transverse to the direction of field propagation. Nonlinear and chaotic behavior in coupled diode lasers is also being studied, as are ways to control the onset of chaotic behavior. The same basic concepts of nonlinear dynamics are found in work on atoms and molecules in electric and magnetic fields and in the dynamics of lasers and nonlinear optical interactions.
Laser Cooling and Trapping
Laser cooling is a technique for transferring momentum, and therefore kinetic energy, from an atom or atomic ion to the photons of a laser beam, as described in Chapter 1. Although first proposed in 1975, and first demonstrated in 1978, laser cooling has already been used to produce what some news stories have called the coldest particles in the universe. The lowest temperatures, in three dimensions, that have been demonstrated thus far have been limited by the recoil of the atom from single-photon emission. This "recoil limit" is of the order of 1 mK for optical transitions. At this temperature, atomic velocities are of the order of 1 cm sec-1, and the corresponding de Broglie wavelengths are several hundred nanometers. Accessing this temperature regime, coupled with the parallel development of methods for trapping atoms and ions by their interaction with external fields, has enabled new and far-reaching opportunities for scientific investigations and technological applications.
A fundamental limitation to high-accuracy atomic spectroscopy stems from the motion of the atoms. Laser cooling reduces the importance of all motional broadening mechanisms, including the Doppler effect and the effect of a finite interrogation time, allowing development of improved atomic clocks. The most thoroughly investigated approach to date uses a microwave transition between hyperfine levels of a cloud of laser-cooled and trapped atomic ions. The "atomic fountain" described in Chapter 1 provides another promising approach. An exotic variant of the fountain in which the atoms fall onto a parabolic mirror and are reflected back has been examined. This "atomic trampoline" may allow for indefinite interrogation times without significant perturbations to the energy levels of the atom. Several groups have also succeeded in trapping and cooling a single atomic ion and are attempting to realize an optical frequency standard that would have a potential accuracy, limited by the second-order Doppler effect, of 1 part in 1018.
Dramatic improvements in many of the tests of fundamental laws and symmetries described above have been made possible through the use of laser cooling and trapping. For example, the mass of trapped antiprotons was recently compared to the mass of the proton via ion cyclotron resonance. This precise measurement is thought to be one of most sensitive tests of the CPT theorem for baryons, which says that any Lorentz-invariant field theory must be invariant under the combined operations of charge conjugation C, parity inversion P, and time reversal T. Laser-cooled ions have already been used in experiments to test
local Lorentz invariance and possible nonlinear corrections to the Schrödinger equation with unprecedented sensitivity. Tests of general relativity, such as the gravitational red shift effect, will also become much more sensitive using the increased accuracy that cooling and trapping will bring to atomic clocks.
Another major area enabled by laser-cooled and trapped atoms involves quantum collective effects. When indistinguishable particles are cooled and compressed until their de Broglie wavelengths are comparable to the mean particle separation, the quantum statistics of the particles will affect the behavior of the entire system. If the particles have integral spin, they will obey Bose-Einstein statistics and may undergo a Bose-Einstein phase transition at low temperature in which a significant fraction of the particles can occupy the macroscopic ground state of the system. Researchers have come tantalizingly close to achieving the goal of observing this effect in a weakly interacting system in the case of magnetically trapped, spin-polarized hydrogen atoms. There are also efforts in several laboratories to laser cool either magnetically trapped lithium or cesium atoms to the necessary temperature and density conditions. These atoms are made up of an even number of spin-1/2 subatomic particles and therefore have integral spin as a composite particle. In these experiments, optical probing of the atoms may provide insight into the dynamics of the condensation process and a means of detecting the atoms in the Bose-condensed phase. It is still an open question whether these atoms remain a gas at absolute zero temperature or will coalesce into a solid or liquid state. It may also be possible to create a weakly interacting degenerate Fermi gas by laser cooling atoms with half-integral spin, which obey Fermi-Dirac statistics.
Laser cooling techniques are being used in conjunction with "particle optics," as discussed in Chapter 1, to produce atomic beams of unprecedented intensity. Lasers can collimate the atomic beam emerging from a source and focus it to dimensions of the order of several tens of micrometers. These intense atomic beams will find applications in atomic collision experiments as high-flux sources for loading atomic traps and may be useful for lithography and nanoengineering (Figure 2.2).
Experiments involving single trapped atomic ions have also permitted fundamental investigations of the nonclassical nature of photon statistics. By isolating the resonance fluorescence from a single laser-cooled and trapped ion, experimentalists were able to observe electron "quantum jumps" between energy levels of the ion. Also, by recording the correlation functions between the photons emitted by a single emitter, the nonclassical effects of "photon-antibunching" and sub-Poissonian photon statistics were observed.
The observation of "Coulomb clusters" is an example of the far-reaching impact of laser cooling and trapping techniques in other areas of physics. E.P. Wigner predicted that a nonneutral plasma would undergo a phase transition from a gas to a liquid or a solid when the ratio of the Coulomb potential energy to the kinetic energy of the gas is increased to a value of approximately 100.
Although many attempts had been made to achieve the required conditions, it was not until laser cooling was brought to bear on the problem that success was achieved (Figure 2.3). Stable "solid" structures were observed with between 2 and approximately 30 ions. These "clusters" assume shapes that minimize the free energy of the system. Larger clouds containing hundreds of thousands of ions have been observed to form concentric rings in which the ions within a ring exhibit liquid behavior while the rings themselves remain quite stable.
Interactions with Surfaces
Interactions of atoms, molecules, and ions at surfaces of liquids and solids have long withstood attempts at detailed understanding, but new technologies—both experimental and theoretical—have opened the door to dramatic advances during the past decade. Much of this progress has resulted from developments in atom-ion beam technology and from advances in ultrafast lasers. Recent work has provided detailed information about atomic and molecular interactions with surfaces, information that is being exploited to improve the understanding of heterogeneous catalysis and to implement new technologies such as X-ray lasers
and fusion reactors and to develop novel methods for the thin film deposition and semiconductor etching important for device fabrication. Ultrafast laser techniques have been applied to monitor the relaxation rate of excited adsorbates and to study charge transfer processes at electrode surfaces and between photoexcited adsorbates and the surface. Such information is crucial to the development of new photochemical synthetic methods at interfaces and new lithographic techniques.
Surface studies, especially those at the quantum-state-resolved level, provide new challenges for theory in an area that lies at the interface between atomic and molecular theory and condensed matter theory. Questions concerning the nature of molecular adsorption and the dynamics of particle-surface interactions remain to be answered.
Studies of particle-surface interactions have resulted in the observation of new atomic species, including so-called "hollow atoms." These are formed by resonant neutralization of slow, highly charged ions incident at grazing angles on a metal surface and comprise essentially neutral atoms having most of their electrons in high-lying Rydberg levels. These highly excited atoms are very
short lived and decay by Auger processes. The successive filling of inner shells is a true dynamical many-body process for which only classical statistical models are currently available. For large angles of incidence the incident ions penetrate the surface, and effects due to screening by metallic electrons become important, allowing study of the behavior of the excited states in the presence of screening. Basic questions remain to be answered, however, concerning the role of electron correlation in the formation of hollow atoms, the positions and widths of multiply excited resonances, and the nature of atomic states in the vicinity of a surface.
The study of arrays of atoms and molecules adsorbed on the surface of atomic or molecular liquids and solids is important for many applications. The continued development of electron spectroscopic techniques, often facilitated by the ready availability of synchrotron radiation, has allowed these structures to be probed at a level of detail never before possible. New optical spectroscopies, such as second-harmonic generation, have probed the liquid-solid interface at a molecular level for the first time. Many of these investigations have been directed toward establishing direct correlations between the chemical and physical information obtained from laboratory experiments carried out under single-collision, ideal conditions on crystalline surfaces and the corresponding reactivity in practical systems involved in heterogeneous catalysis. They have resulted in substantial new insights into catalytic reaction mechanisms, which have the potential to form the basis for improvements in catalyst performance. The emphasis is shifting from analysis of surface structure and properties to their control.
A profound development in the area of surface studies is the scanning tunneling microscope (STM) and the related atomic force microscope (AFM), which can image a surface with atomic resolution, thereby enabling the position (and orientation) of adsorbed atoms and molecules to be determined. These new instruments have been used to identify the active site on which a surface chemical reaction occurs. Such microscopic information is crucial to the formulation of a more general theory for catalysis. Tunneling microscopy has also been used to measure the electronic spectrum of single adsorbed molecules and to promote their reaction. Recent research has demonstrated that the position of an adsorbed atom or molecule can be manipulated using an STM and its derivatives. This may allow the building of intricate nanostructures on a surface and provide unparalleled control of surface structure and composition.
Enabling Other Fields of Science
The importance and impact of AMO science are in significant measure due to the critical role it plays in enabling many other scientific disciplines. Experimental techniques and measurement procedures that derive from AMO studies are widely used in other areas of science, including astrophysics, space science, atmospheric and environmental science, plasma physics, exotic atoms and nuclear
physics, surface and condensed matter physics, and the biosciences. Information regarding the properties of atoms, ions, molecules, electrons, and photons is critical to modeling and understanding many physical environments.
Apart from cosmic rays, all the information we have about the universe beyond the solar system is brought to us by photons. Astronomy relies on the interpretation of the distribution in frequency and intensity of the photons that are emitted by astronomical objects and detected by ground- or space-based telescopes. Even information about the earliest stages in the history of the universe before any nuclei, atoms, or molecules existed, before the galaxies and stars had formed, is carried to us by photons. The differential microwave radiometer on the Cosmic Background Explorer satellite tells us about conditions in the early universe by measuring the photon intensities at millimeter wavelengths.
The processes that create the photons and modify them in their journey to Earth through intergalactic and interstellar space from their distant origins belong to the domain of AMO science, as do the instruments that monitor these photons and measure their spectra. The photon distribution with wavelength is used to define the kind of object and to distinguish among different kinds of galaxies, stars, pulsars, jets, stellar winds, supernova and supernova remnants, nebulae, masers, protostars, and molecular clouds. A major triumph of astronomy has been the placing of stars in an evolutionary sequence constructed from analysis of stellar spectra that change as stars are born, grow old, and die.
To achieve a comparable understanding of other astronomical entities requires a comprehensive grasp of AMO science supported by an extensive database of atomic, molecular, and optical parameters. Providing these data is a major challenge. Experimental laboratory studies are essential for high precision and to test theoretical procedures, but the huge and growing volume of data that are needed can be provided only by theoretical calculations. Data are not enough. A deeper appreciation of atomic, molecular, and optical processes as they occur in the exotic conditions found in astronomical environments is also essential and can be gained only by basic experimental and theoretical AMO studies. We can also learn about atomic, molecular, and optical properties from investigations of astronomical phenomena. Despite the enormous differences in spatial and time scales, the fundamental processes in laboratory and astronomical plasmas are the same.
Lasers are becoming increasingly important in astronomical measurements. They are used to accurately measure and control baselines in coherent interferometry in the microwave region. Dust grain distributions around stars have been measured by using a carbon dioxide laser as the local oscillator for heterodyne interferometry. Laser-produced artificial high-altitude light sources have been used recently in adaptive optical wavefront correcting systems. The image quality
in ground-based astronomy has been severely limited by the wavefront distortion introduced by atmospheric turbulence. If a test wave is available, this distortion can be measured and corrected for by using a deformable optic allowing large ground-based telescopes to have resolution similar to that of comparable space-based systems. Over a limited range of the sky, high-magnitude stars can be used to provide the test wave. For most observations, however, the test wave must be generated by using laser scattering from aerosols in the lower atmosphere or fluorescence from sodium atoms in the mesosphere pumped by a laser source of sodium resonance radiation. Maximizing the sodium fluorescence requires a detailed understanding of the dynamics of the atomic interaction with the laser radiation (Figure 2.4). Hyperfine splitting, Doppler broadening, coherent excitation, saturation, optical pumping, and radiation pressure are all important.
In the next decade, new windows will be opened through which the universe will be observed. Ground-based and satellite-borne telescopes supported by powerful instruments for photon detection will provide a vast array of spectral data at high resolution in the radio, millimeter, submillimeter, infrared, optical, ultraviolet, and X-ray regions of the electromagnetic spectrum. The increased sophistication and detail of theoretical models of astronomical events and the
quality of the observational data will place severe demands on AMO science that must be met if the full potential of astrophysical research is to be realized.
The central role of AMO science is evident in a broad range of significant astronomical questions. For example, the helium abundance in the primeval universe constrains the number of neutrino flavors and the age of the universe. The ratio of deuterium to hydrogen (D/H) provides a measure of the baryon density in the early universe. This ratio can be obtained directly in the solar neighborhood from the absorption of starlight by deuterium and hydrogen atoms. To determine the D/H ratio over wider regions of the galaxy, measurements of deuterated molecules like DCO+ and DCN can be used. Deuterated molecules undergo extensive chemical fractionation that enhances their abundances. To extract the D/H ratio requires detailed models of the fractionation, but the D/H ratio inferred from the observations is consistent with the value obtained for the solar neighborhood, suggesting it is a characteristic of the entire galaxy.
The D/H ratio in the universe is thought to diminish in time because of nuclear burning. Observations of DCO+ in external galaxies coupled with a precise description of the atomic and molecular processes that lead to fractionation would test the assumption that the D/H ratio decreases with time. If it does, the D/H ratio measured for the galaxy establishes that baryonic matter cannot close the universe. It is possible that with improved infrared technology the deuterated hydrogen molecule could be seen in external galaxies and provide an additional measure of the D/H ratio, but a sophisticated interpretation of the emission would be necessary.
Recently, substantial advances have been made in building a theoretical description of molecular processes in interstellar clouds to the point where the molecular composition can serve as a chemical clock for determining the age of a cloud so that clouds can be placed into an evolutionary sequence. The changes in chemical composition that occur during the birth of a star have been explored, and diagnostic probes have been worked out in which the emission line widths of specific molecular species are used to investigate the phases of gravitational collapse occurring deep inside molecular clouds and hidden from view except through molecular emission lines at millimeter and radio wavelengths. To carry through detailed analyses of any reliability, a comprehensive set of molecular reaction rate coefficients at temperatures ranging from 5 to 2,000 K is needed. Greatly improved data on X-ray spectra are expected to flow from spacecraft launched in the next several years. Except in a few cases, the collision cross sections, recombination rate coefficients, and radiative transition probabilities are currently available only as estimates of limited reliability.
As a final example, supernova SN 1987a, the brightest supernova observed since the invention of the telescope, has been of enormous importance in astronomy. Observations over the years since the progenitor star exploded have yielded a store of information about stellar evolution, explosive nucleosynthesis, and
stellar dynamics, all obtained by analyses of the spectra. After the first few days, the supernova became in effect a dynamically evolving laboratory of atomic and molecular physics, and the interpretation of the data has raised many questions in atomic and molecular physics that have to be addressed before reliable conclusions can be drawn about the dynamical evolution of the ejecta and the interactions with the interstellar medium in which the explosion occurred.
In the context of this report, space science refers to the study of the solar system. It considers the interplanetary medium, the planets and their atmospheres, and comets, asteroids, and small bodies. AMO science provides the atomic and molecular energy levels, optical transition probabilities, cross sections, and rate coefficients that are the central parameters in quantitative models.
A recent example of the important role of AMO science in space is the discovery of the molecular ion H3+ and the explanation of unidentified lines in the Jovian aurora. Theoretical calculations of the potential energy surface and the resulting energy levels, supported by laboratory studies of selected transitions, provided detailed predictions of the emission spectra of H3+. These corresponded to observed, but previously unidentified, Jovian infrared emissions, and these emissions from H3+ are now used to provide images of the Jovian aurorae.
Comets may carry the imprint of their origin in the interstellar medium prior to the formation of the solar system 4.5 billion years ago. The analysis of cometary data is based on elaborate models of the effects of atomic and molecular processes and solar radiation on material released as the comet more closely approaches the sun.
The study of planetary atmospheres and the search for the reasons why the atmospheres of the planets and their satellite moons have evolved in different ways provide insight into the mechanisms that operate in Earth's atmosphere and assist in developing an understanding of such critical issues as the warming of the atmosphere due to the increase in carbon dioxide and the destruction of ozone due to the industrial release of chlorofluorocarbons. These processes are discussed further in the next section. Comparisons of the atmospheres of Earth, Mars, and Venus demonstrate the sensitivity of atmospheric composition to perturbations.
The escape of gases from the planets into interplanetary space has a major influence on atmospheric composition. Escape can occur through a variety of atomic and molecular processes. On Mars, nitrogen loss is driven by dissociative recombination; on Venus, helium loss is driven by charge transfer. The ionospheres of all the planets are caused by the absorption of solar radiation in photoionizing events and by ionization through solar cosmic ray impacts. Aurorae are a feature of planets in which excitation is caused by the impact of fast electrons and ions, accelerated in the magnetosphere. The atmospheres respond
to radiation with a great variety of molecular reactions, and the construction of realistic models of atmospheric behavior is impeded by the lack of an accurate database and also by an insufficient grasp of the enormous variety of atomic and molecular processes that occur.
Space physics differs from astrophysics in that local measurements are possible using spacecraft. Satellite-borne instrumentation has yielded basic data on planets, comets, and the magnetosphere. This instrumentation is in many cases directly descended from apparatus developed for laboratory-based AMO studies.
Doppler shifts and line widths of molecules in space and planetary atmospheres have been measured using heterodyne interferometry. Carbon dioxide and lead-salt lasers have been used to detect carbon dioxide in the Martian atmosphere. Studies have been made of atmospheric wind variations, and information on altitude dependence has been obtained from observations of the line profiles. Laser sounding systems are also being developed as onboard instruments for planetary probes.
Atmospheric and Environmental Science
There is national awareness of the importance of protecting Earth's atmosphere, as reflected, in particular, in concern with global warming and with depletion of Earth's ozone shield. Global warming is the predicted change in the global temperature resulting from a steadily increasing level in the atmosphere of carbon dioxide and several other trace gases from anthropogenic sources. Although the gaseous contaminants occur in small amounts, their role in absorbing and radiating long-wavelength infrared energy has the potential to effect large changes in Earth's temperature. The actual changes that might occur are governed by a complex series of AMO processes. A similar situation holds in the case of ozone depletion in the stratosphere. A thin shield of ozone molecules protects life on Earth's surface from the destructive effect of solar ultraviolet radiation. There is evidence that anthropogenic sources are affecting the level of ozone in the stratosphere. Limiting these sources of damaging chemicals significantly affects the economies of both the United States and other countries. Thus an accurate understanding of the development of this problem has major implications for society and national economic planning.
Research directed at understanding our natural environment proceeds along three parallel fronts, in each of which AMO science plays a key role. The first involves the development and utilization of computer models to analyze the effect of anthropogenic physical and chemical perturbations and to predict the response of environmental systems to possible future variations in these perturbations. The second research front focuses on laboratory studies designed to isolate and characterize individual chemical and physical processes and to accurately quantify their controlling parameters. This includes measuring quantities such as rate constants, cross sections, line shapes, scattering and extinction albedos,
and other basic physical parameters that are at the core of AMO physics and closely related fields. Of course, not all of the relevant microscale parameters are determined by experiments. Theoretical molecular structure and molecular dynamics studies can often yield valuable information, particularly for species and/or physical conditions that pose experimental difficulties. The third research front is the development of sensitive physical and chemical sensors and their use in field measurements to quantify the current state of critical environmental parameters such as trace species concentrations, temperatures, and phase distributions. Measurements are also essential for characterizing and quantifying rates of change in key parameters such as fluxes due to heat and mass transfer, chemical reaction, evaporation, nucleation, condensation, and solidification. AMO science is key to many atmospheric sensing methods.
The three lines of environmental research described above are closely linked. Laboratory research supplies critical information (atomic and molecular parameters) needed to refine and extend the computer models. It also provides field instrument developers with the information they require to quantify advanced sensor performance. Sensitivity analyses of model results can, in turn, identify the microscale parameters that most need further investigation. Models are also used both to design and to analyze the data from field measurement programs. The environmental quantities obtained in field measurements, such as the temporal and spatial variations of trace species concentrations or temperatures, provide the yardstick to judge a model's accuracy and predictive capability.
The trace species content of the global atmosphere is changing at a rapid rate, particularly compared to geologic time scales. Numerical computer models that attempt to quantify the flux of trace species into and out of the atmosphere, the fluxes of infrared, visible, and ultraviolet light through the atmosphere, the rates of atmospheric chemical transformation and physical state change (such as cloud formation, evaporation, and precipitation), and the transport of atmospheric mass (such as by wind or diffusion) are the repository of all we know quantitatively about atmospheric model assessments.
The most sophisticated of these models rely on a wealth of AMO physical detail. For instance, as sunlight penetrates the atmosphere, its ultraviolet component initiates critical photochemical processes by photodissociating O2, O3, HO, NO, ClO, NO3, and a variety of other atmospheric species. Ultraviolet and visible light is also scattered by cloud droplets, clear air aerosols, and Earth's surface. All of these absorption and scattering processes depend strongly on the wavelength of the light and the local chemical and physical state of the atmosphere. Models of the atmospheric transport of solar radiation require sophisticated theoretical techniques to allow adequate representation of the effects of physical optics such as scattering and absorption by atmospheric droplets and particles. These scattering models are combined with the quantitative molecular spectroscopy necessary to represent the effect of gaseous species in order to accurately reproduce the temperature field of the upper atmosphere and the photochemical
driving force for chemical change in both the upper and the lower atmosphere. However, even the most sophisticated solar radiation transport model is only half the atmospheric radiation transport story. The outgoing ''earthshine" at infrared wavelengths is critical to temperature fields at Earth's surface and in the lower atmosphere. A full model must also adequately represent the absorption due to at least a half a dozen key infrared active molecular trace species, all with different altitude profiles and with pressure-sensitive infrared line shapes. Furthermore, the ability of clouds of different types to absorb and re-emit infrared radiation depends critically on their altitude, optical depth, cloud droplet size spectrum, and physical state as well as their temperature relative to that of Earth's surface and other cloud layers.
Once an adequate representation of radiation transport has been achieved, chemical transformations in both the gas and the condensed phases (due to heterogeneous interactions between trace gas species and cloud droplets and/or aerosols) must be considered. Models that include several hundred homogeneous gas-phase and heterogeneous condensed-phase reactions are no longer unusual. In addition, heat and mass transport, both within the atmosphere and at the lower (Earth) and upper atmospheric boundary layers must be properly parameterized. Finally, modelers may add in the nucleation and growth of aerosol particles from high-vapor-pressure substances produced by atmospheric chemistry, which can, in turn, serve as condensation sites for cloud droplets and ice particles, leading to substantial perturbations on both heat flux and mass transport in and around the cloud and significant chemical perturbations via heterogeneous reactions. It is the balance of all these AMO processes that determines the progression of global warming under the influence of the greenhouse effect. In the same way, a complicated photochemistry determines the ozone depletion in the stratosphere.
The tools, techniques, and data provided by AMO science are of vital importance in addressing global atmospheric environmental problems such as stratospheric ozone depletion, greenhouse gas buildup, and acid deposition and tropospheric oxidative capacity. Atmospheric modeling and the interpretation of atmospheric diagnostics require accurate knowledge of molecular and atomic energy levels, transition probabilities, photodissociation and photoionization cross sections, reaction rates, and collision cross sections. The available data are incomplete, especially as concerns infrared and ultraviolet transitions in trace molecules and free radicals and reaction rates involving free radicals. An expanded database is essential to advances in atmospheric modeling for meteorology and the prediction of climate change and the effect of local and global pollutants.
AMO science plays a pivotal role in the study of plasmas. The formation and evolution of laboratory plasmas depend on microscopic processes that influence
the material equation of state and determine the energy production and transport. Also, atomic and molecular spectroscopic measurements provide a direct probe of the plasma environment, yielding its temperature, density, velocity, and internal electromagnetic field strengths. The quest for inertial and magnetic confinement thermonuclear fusion demands an extensive database describing atomic phenomena in high-temperature plasmas in which intense electromagnetic fields and shock waves may be present. Conventional energy production devices, such as magnetohydrodynamic generators, have spurred extensions of our understanding of complex atomic interactions in external electromagnetic fields. Modern defense programs rely increasingly on our knowledge of high-energy-density plasmas. In laboratory X-ray laser studies, our ability to model the phenomena satisfactorily is sometimes limited by an incomplete understanding of subtle quantum electrodynamic effects on the shapes of spectral lines. Low-temperature plasmas are fundamental to commercial and residential lighting. Plasma processing of materials is of vital importance to several of the largest manufacturing industries. It is indispensable for manufacturing the very-large-scale integrated microelectronic circuits used in computers, communication equipment, and consumer electronics. Plasma processing of materials is important in the automotive, aerospace, steel, and biomedical industries. A severe limitation in the design of plasmas for material processing is the lack of a detailed understanding of the atomic and molecular processes that occur. Advances in technology are providing new capabilities for the study of atomic processes in plasmas. Magnetic confinement fusion experiments can attain long-lived plasmas at temperatures of tens of kilovolts through the careful control of atomic processes involving impurity ions and the interactions of the bulk hydrogen plasma with the walls of the device. Large laser facilities have compressed target materials to hundreds of times their normal solid densities, reaching conditions heretofore found only in astrophysical objects. In addition to the fundamental attraction of producing atoms in strong electromagnetic fields, short-pulse lasers offer opportunities to study warm, dense plasmas without the complications of hydrodynamic motion that inevitably occur on longer time scales.
The advent of supercomputers has permitted theorists to make more accurate calculations of individual processes as well as integrate many atomic processes into comprehensive models of plasma behavior. In several instances, plasma modeling has progressed from a set of descriptive methods for explaining crude observations to a set of procedures for the design of practical devices. A full understanding of how and to what extent a plasma environment actually modifies reaction rates is lacking. Indeed, after much study, even for the relatively simple processes of direct collisional excitation or ionization, substantial disagreement still remains between rate coefficients measured in a plasma and those calculated from isolated atom cross-section data.
Complications arise because the importance of a particular process in a
plasma is affected by the state of the plasma itself. For example, the rate at which collisional ionization of a given ionic species occurs is determined by the populations of its excited levels, which in turn depend on the velocity distributions of plasma electrons and ions, or the ambient radiation, and sometimes on the presence of an intense electric or magnetic field. In dense plasmas, the simple picture of atoms in a plasma breaks down, and methods similar to those developed in condensed matter physics and the physics of liquids must be invoked.
Numerous opportunities and challenges for AMO science can be identified in plasma physics. At low temperatures, there is an immediate need for accurate measurements of the complex molecular processes that occur in partially ionized plasmas. This includes both surface phenomena and interactions by which complex molecular structures are formed in the plasma. Intense, short-pulse lasers offer the opportunity to study transient phenomena in warm, dense plasmas on time scales that are short in comparison with time scales for hydrodynamic motion and many plasma-cooling processes. High-energy-laser facilities will be capable of compressing and heating matter to conditions hitherto found only in astrophysical environments, thus also providing new insight into cosmic phenomena. The increasing availability of sophisticated computers will allow accurate calculations to be performed for a wide range of processes. The quality of plasma modeling will be enhanced and will provide the first estimates of the importance of processes that could not be included in previous models. New formulations will be developed to describe the complex many-body processes discovered in the past decade. As new fundamental ideas are developed, it will be necessary to provide workable approximations for use in practical applications. Of particular importance is the development of simple models that explicitly include the interaction of the atomic and plasma environments. New directions in computer design, especially of massively parallel computers, present the opportunity to revisit the very foundations of computational atomic and plasma theory.
Exotic Atoms and Nuclear Physics
AMO physics can probe directly the properties of nuclear matter. As discussed earlier, experiments to test fundamental symmetries such as parity nonconservation are often based on precision atomic and molecular spectroscopy. Their interpretation makes use of refined atomic and molecular many-body calculations whose accuracy can be assessed by comparison with other atomic and molecular properties. Atomic parity nonconservation experiments may provide a unique method to measure neutron distributions in heavy nuclei. Experiments on hyperfine structure are a major source of information on nuclear magnetic dipole and electric quadrupole moments, nuclear sizes, and deformations. They require sophisticated atomic calculations to derive quantitative information about nuclear properties.
Collisions at low energies between systems of high nuclear charge z, such as uranium nuclei, are a potential source of information about quasi-molecules with transient nuclear charges in excess of the inverse of the fine-structure constant and could provide a stringent test of quantum electrodynamics. Unexpected phenomena have already been found in the form of electron-positron pair production arising from the decay of neutral particles.
The study of exotic atoms and molecules involving the states of negative elementary particles bound to nuclei is a rich and continuing area of research, giving insight into nuclear structure and hadron interactions. In exotic atoms and molecules an electron is replaced by another negative elementary particle, the most common being the muon (µ¯), pion (π¯), kaon (K¯), and antiproton (p¯). All except the antiproton are unstable but have lifetimes that are long in comparison with many interesting chemical time scales. The relatively long-lived muon is uniquely important in that, like the electron, it has no strong (hadronic) interactions and provides both fundamental and practical insights into the structure of nuclei.
Muonic atoms have been studied for many years, but modern facilities have made this work much more productive. Careful measurements of the energies of cascade X-rays have provided much information on the sizes and shapes of many nuclei. Even more information can be obtained by using polarized atoms. Recently, large polarization of the muonic helium atom (30%) has been achieved by adding a small amount of rubidium vapor to a gaseous helium target. The rubidium is laser polarized and in turn polarizes the electron in the 4He-µ¯-e¯ system by exchange, which then polarizes the muon. This system will be used with 3He to measure the recoil asymmetry of muon capture to determine the induced pseudoscalar coupling constant (this tests the partial conservation of axial current and our understanding of hadronic effects on the weak current).
Muons can also be used to catalyze fusion reactions. In this process, two hydrogenic nuclei, for example, a deuteron and a triton, are bound to form a hydrogenlike molecular ion but with the internuclear distance reduced by a factor of over 200 owing to the large mass of the muon (207 times greater than the mass of the electron). This suppression of the Coulomb barrier allows fusion to occur rapidly (on time scales of a picosecond) and presents a method to achieve nuclear fusion without the high-temperature and confinement difficulties inherent in plasma fusion concepts. However, although a yield of ~150 fusions per muon has been experimentally observed in deuterium-tritium mixtures, this is still well short of that required for energy production. The number of fusions is limited by the cycle rate (compared to the muon lifetime) as well as the probability that the muon "sticks" to the helium nucleus produced by nuclear fusion. The precise calculations of both the binding energy of the deuterium-tritium-muon molecule and the sticking probability have been challenging and have required the development of new theoretical techniques. Variational methods of unprecedented accuracy have been refined for the three-body problem involving long-range
interactions and are expected to find application in other areas. There remains a significant disagreement between the theoretical and the experimental values of the sticking fraction whose resolution might reveal some important new physics.
The low-energy antiproton ring (LEAR) at the European Center for Nuclear Research (CERN) is now capable in principle of producing antiprotonic atoms, and a number of important experiments are anticipated. Because of the large mass of the antiproton, states with extremely high principal quantum numbers will be populated. Topics of interest include formation and cascade at extremely high principal quantum numbers, the effect of the strong interaction of the antiproton with the nucleus, annihilation processes, and static particle parameters such as the mass and magnetic moment of the antiproton determined by using atoms in sufficiently high states that the strong interaction is negligible.
The lifetime of an antiprotonic atom in a high Rydberg state is usually limited by internal or external Auger processes. A novel class of antiprotonic helium states has recently been discovered that suppresses Auger quenching. The system consists of 4He-p¯-e¯ in a highly excited moleculelike configuration. In pure helium, these states have been observed to live for microseconds, much longer than the subpicosecond lifetimes normally associated with Auger processes. Further experiments are under way to detect visible light produced in the deexcitation process and to pump the metastable atoms with lasers, perhaps prolonging this lifetime even further. It may even prove possible to catalyze antihydrogen production via reactions of the metastable atoms with positrons or positronium.
Surface and Condensed Matter Physics
A rapidly expanding arsenal of analytical methods based on particle-surface interactions has been developed for use in surface and materials characterization. Techniques based on electron and atom diffraction are used to examine surface order and structure. Auger spectroscopy and ion backscattering are employed to determine surface composition and monitor contamination. Recently, a variety of spin-sensitive spectroscopies have been implemented that, coupled with advances in the technology to grow thin epitaxial films and engineer artificial structures, have made possible the observation of a variety of novel magnetic phenomena at surfaces and interfaces. One especially powerful technique developed to examine magnetic microstructure is scanning electron microscopy with polarization analysis (SEMPA), in which a highly focused electron beam is directed at the surface of interest and the polarization of the ejected secondary electrons is measured. This approach allows high-resolution studies of domain structure and boundaries with applications in magnetic recording (Figure 2.5). Small-angle ion-surface scattering also provides a powerful probe of surface magnetic properties. Using this approach, it has been shown that surface magnetic properties frequently differ from those of the underlying bulk and that
short-range magnetic order persists even above the surface Curie temperature. Surface magnetic properties have also been examined by investigating spin dependencies in the interaction of thermal energy electron-spin-polarized rare-gas metastable atoms with surfaces. This technique is particularly surface-specific because the incident atoms do not penetrate the surface.
The advent of ultrashort pulse lasers and time-resolved optical measurement methods has contributed substantially to condensed matter physics and has enabled the direct study of dynamical phenomena in solids and at surfaces on time scales typical of the phenomena themselves. Accordingly, much new information has been obtained, primarily in the areas of surface dynamics, nonequilibrium heating of metals, carrier dynamics in semiconductors, and relaxation phenomena in amorphous materials.
Femtosecond lasers have played a key role in the area of surface dynamical processes. A long-standing question about whether surface processes such as desorption are primarily thermal or electronic in nature has been resolved by time-domain observations of the desorption event. The fast time scale on which
desorption is observed to occur has revealed new desorption mechanisms and has stimulated interest in the role of the substrate nonequilibrium electronic properties in surface dynamical events. Also of interest is how the presence of a nearby surface affects the lifetimes of excited states of atoms and simple molecules. To that end, the lifetimes of image states at metal surfaces have been measured directly, and the extremely short lifetimes observed point to the importance of electronic deexcitation mechanisms in, for example, chemical processes occurring at surfaces.
The nonequilibrium electronic properties of bulk solids is an active research area. In the first few hundred femtoseconds after a laser pulse strikes a solid, the electronic temperature of the solid is highly elevated, but the solid remains vibrationally cold. In times corresponding to approximately the inverse of the highest relevant phonon frequency, the lattice temperature begins to rise, and subsequently the hot electrons reach thermal equilibrium with the solid. The distribution and cooling of hot electrons have been studied optically by femtosecond thermomodulation and, more directly, by observing photoelectron emission on this time scale. These studies have provided a new and direct method for measuring electron-phonon coupling constants in solids, important in understanding superconductivity. They have also resulted in the first detailed observations of thermalization dynamics in metals, directly yielding electron distributions during nonequilibrium cooling.
In semiconductors, carrier thermalization of highly excited electrons and holes has been studied extensively, but fundamental questions remain. The intraband thermalization via phonon emission has been elucidated in gallium arsenide, and electron-phonon interactions in the space-charge region near semiconductor surfaces have been investigated. Many-electron effects are important in the screening and initial thermalization of hot electrons and holes but are still poorly understood, and further theoretical and experimental efforts are required. In amorphous semiconductors and glasses, relaxation effects can occur on widely varying time scales, owing to electron localization effects and distributions of low-energy trap states. Time-resolved investigations of carrier relaxation in these systems have provided an important method for establishing the presence of mobility edges and can probe the interaction between localized and extended states in these materials.
Biosciences—Mapping the Human Genome
The development and utility of optical tweezers have been noted in Chapter 1. Here the focus is on the use of optical techniques for mapping the human genome, because these promise greatly increased sequencing rates. The human genome is composed of approximately 3 × 109 base pairs organized into 23 continuous subsequences (chromosomes). With overlap requirements (fragments are sequenced and overlap regions used to match fragments) and duplications
necessary to remove ambiguities, it is estimated that about 5 × 1010 base pairs of raw sequence must be accumulated. In order to map the genome, it is necessary to develop a fast process for recording base pairs in DNA. An optical technique is being developed that will increase the raw sequencing rate by a factor of 1,000 over current, state-of-the-art automated sequencing techniques. The approach is to synthesize a complementary strand of DNA using fluorescently labeled nucleotides in the reaction mixture. Each of the four bases (A, C, T, and G) is tagged with a nucleotide having a different fluorescent dye. The fluorescently tagged DNA fragment is attached to a microsphere. The microsphere is placed in a flowing sample stream, and the individual, fluorescently labeled bases are cleaved from the DNA fragment sequentially by an oxonuclease and are identified by photon burst fluorescence as they pass downstream. The light forces associated with a focused laser beam are sufficient to hold and move individual microspheres and allow manipulation of individual DNA strands.
In another optical approach to the Human Genome Project, genome sequencing using direct imaging X-ray color holography is being explored. This technique is similar to fluorescence labeling, but the four bases in a segment of DNA are tagged with different heavy atoms, thus causing only small structural alterations to the configuration of the original (unaltered) bases. The segments are placed on a 100-angstrom (Å) carbon foil, and a tungsten sphere is positioned adjacent to the DNA segment. Irradiation with 5-Å radiation of sufficient coherence length can produce a holograph. Scattering from the tungsten sphere produces the reference beam, which interferes with the scattered beam from the DNA. A 1-micrometer coherence length is sufficient and can be obtained from an undulator. The scattered radiation (hologram) will be read by a two-dimensional, X-ray-sensitive charge-coupled device and the image of the DNA reconstructed.
THE NATION'S MEASUREMENT TECHNOLOGY
Among the ways a field of science can have importance are through its scientific interest, its impact on other fields of science, and its impact on technological or societal issues. The first two of these are briefly examined above. The third is examined here and in the following section, beginning with a look at measurement. The measurement standards and methods in use today are based primarily in AMO science, and a large fraction of modern measurement methodologies and instruments originate in AMO science.
Accurate and precisely interrelated measurements are essential to equity in trade, quality control in manufacture, access to the global marketplace, and the progress of science itself. In the second half of this century, we have witnessed a
significant paradigm shift, led by AMO science, away from the traditional, hierarchical, and artifact standards to standards based on more sharply definable and independently realizable atomic and molecular parameters.
Earliest historical records indicate use of measures and standards primarily for equity in trade. For a local trading region and the limited accuracy required, the arbitrariness and inconstancy of the standards had little impact. However, with increasing world trade greater standardization was required, and this was provided by the Treaty of the Meter, which came into worldwide effect only a little over a century ago. This major advance in measurement science and standardization was, from a present-day viewpoint, flawed in its radical dependence on a singular collection of artifacts. Among these, one was designated as the basis for each quantity, as, for example, a particular meter bar and one of the kilogram replicas. These were (and in the case of the kilogram, still are) maintained under secure and stable condition at the Bureau des International Poids et Mesures (BIPM). The global measurement system was then realized by distribution of replicas, each normalized to the BIPM standards and periodically recalibrated. In the national standardizing laboratories such as the National Institute of Standards and Technology (NIST; formerly NBS) in the United States, these metrological tokens were further transferred to "working" standards against which subsidiary replicas used in commerce, science, and industry were calibrated and the results thus disseminated. This intricate structure met commercial, scientific, and industrial needs for a time but is no longer adequate.
In the second half of the twentieth century, a new paradigm has emerged, largely through AMO science. In this, the basic standards are connected as closely as possible to precisely measurable intrinsic atomic (or molecular) properties or to the fundamental constants of nature. The base units are then freely realizable by any laboratory or technical installation having the necessary resources. In practice, of course, not all measurements require ultimate precision and accuracy, so remnants of the hierarchical system remain in recommended realizations of secondary standards. There is a clear trend, however, to develop even the working standards as independently realizable systems, thus immune to the need for calibration by a central authority.
Up to the present, this shift in standards philosophy has been successfully applied and internationally accepted for all principal base units with the single exception of the unit of mass. Only the kilogram remains artifact-based and hierarchically disseminated. Even in the case of mass, however, AMO scientists are taking promising steps toward a possible atomic standard and replacement of this last vestige of artifact-based measurement. The prototype for the new atomic standards paradigm is the standard of time/frequency, now based on the oscillations of an undisturbed cesium atom. Time and frequency are the most accurately measurable of all physical quantities. Accuracies now achieved are better than 1 in 1013, with precisions approaching 1 in 1017. AMO scientists are conducting research that is anticipated to push the ultimate accuracies to parts in
1018 using laser manipulation and cooling techniques described earlier. In 1984 the unit of length was redefined in terms of a fixed (defined) value for the speed of light. Recommended realizations of the "length" definition through stabilized lasers permit length measurement accuracy of about 2 parts in 1010, with length measurements thus becoming a subset of frequency metrology.
Measurement and Instrumentation
The continued advance of science and technology relies heavily on improvements in measurement techniques and instrumentation, and this is an area in which AMO science plays a key and facilitating role. Indeed, many discoveries have resulted directly from improvements in measurement techniques. The ability to observe the world via instruments that extend our limited powers of observation allows us to continually increase our understanding of the universe. In addressing questions of global change, AMO technology now makes possible precise measurement of toxic gases in the environment and of changes in the key components of Earth's atmosphere. In the world of the ultrasmall, the continued improvements in the performance and resolution of microscopes have driven a multitude of discoveries in the biological and materials sciences. In addition, these advances in microscopy have nurtured the development of very-large-scale integration of semiconductor devices on a single chip.
AMO science is central to the precise measurement of time, upon which the synchronization of our complex civilization depends. For example, it is now possible to localize one's position on Earth to an accuracy of a few meters by using atomic clock timing signals from navigational satellites, which may well lead to safer travel and could form the basis of automated highways.
As science advances, as society becomes more conscious of the way it cares for the environment, and as more sophisticated products enhance our lives, more refined measurements and more refined understanding of the phenomena attendant to the event or activity are being seen as necessary and useful capabilities. AMO science is the cornerstone in the infrastructure associated with these advances.
AMO science has in recent years produced a number of enabling technical advances that have led directly to new instrumentation and measurement technology. Some of these advances have resulted in commercial products; others have not yet reached that stage but are almost routinely used by researchers in virtually all fields of physical science and engineering throughout the nation and the world. A few of the more notable AMO technical advances include the following:
Atom interferometry, which may lead to orders-of-magnitude improvement in applied areas such as navigation and geophysics.
High-resolution optical spectroscopy based on lasers, including Raman and other nonlinear optical spectroscopic techniques, with applications ranging
from atmospheric remote sensing of pollutants or other constituents to noncontact temperature measurement in flames.
Ion beam techniques for doping semiconductors, optical metrology for lithographic patterning, and optical surface diagnostic techniques, which are all essential parts of today's modern semiconductor integrated circuit industry.
Ultrashort optical laser pulses for measurement of temporal phenomena that happen on time scales extending into the femtosecond regime with important applications, for example, in the fundamental study of vision and photosynthesis process as well as in modern medical procedures.
Carefully controlled and measured pulsed optical power for use in controlled ablation and evaporation of materials. For example, high-temperature superconducting films are prepared by using high-energy laser pulses. Lower-power pulses scribe and trim integrated circuits and can weld tears in the retina or reshape the cornea.
AMO in Measurement and Sensing for Industry
One of the largest roles of AMO science in industrial applications has been in the area of measurement. On-line monitoring, process optimization, quality control, pollution control, and nondestructive testing simply would not exist without the ability to sense and measure the relevant quantities. For example, manufacture of low-loss, single-mode optical fibers begins with a glass preform that is grown from high-purity chemicals. On-line monitoring is needed during each manufacturing stage. The preform is then drawn into a fiber, and the tight dimensional tolerances required are maintained through real-time measurements with immediate feedback through a control loop.
As another example, commercial instruments combining gas chromatography followed by time-of-flight mass spectrometry of elutants have revolutionized the analysis of complex liquid mixtures of large molecules. AMO research and trained AMO scientists are essential in the invention of customized chemical analytical instruments such as the gas and liquid chromatographs, visible and infrared spectrometers, and mass spectrometers that are used in industry. The design and engineering of such instruments require a thorough training in molecular and optical science at all stages.
Robots in automated manufacturing must have ''eyes" to sense their environment, and this input forms the basis for decisions and actions by the robot. The continual development of new improved eyes, or sensors, is one of the challenges in automated manufacturing, and it is no surprise that fiber optics and lasers have assumed prominent positions in modern sensor technology. Indeed, fiber-optic sensors have been developed to measure a wide range of physical observables and offer the advantages of high sensitivity, inherent immunity to electromagnetic noise, ease of remote placement, usefulness in hostile environments, and small size and low weight.
Increasing use of sensors, both optical and chemical, is critical for the development of automated manufacturing, which is widely seen as necessary for the renaissance of U.S. manufacturing in areas ranging from food and drink preparation to chemicals and high-tech electronics. Innovative sensor development represents an important opportunity for future research in AMO science.
THE NATION'S TECHNOLOGICAL INFRASTRUCTURE AND U.S. ECONOMIC PRODUCTIVITY, COMPETITIVE POSITION, AND SECURITY
AMO science is a significant contributor to the nation's overall technological infrastructure and to its economic health and security. Indeed, it is estimated in Chapter 5 of this report that the products of AMO science have a significant impact on well over 9% of the nation's GNP and are important to an even larger percentage. The many impacts of AMO science in these areas are illustrated by the following examples, which contain a number of recurring themes. For instance, the large amount of AMO data now available describing the properties of atoms, molecules, ions, and photons is essential to understanding and modeling many different environments and systems. The required database, however, is far from complete, and continued research is required to expand it to allow questions of current importance to be addressed. Lasers have enabled major advances in almost every aspect of science and technology. Nonetheless, their application is limited in many instances by barriers of cost and performance. To remove these restrictions, further research and development are required to produce a new generation of lasers that are more efficient, more reliable, and less expensive than those currently available, with wavelength and output characteristics tailored to specific applications.
Industrial Technology, Manufacturing, and Processing
AMO science is central to many industrial manufacturing processes and technologies. Today, manufacturing is a highly technical and automated business requiring advanced methods that must continue to improve and evolve to remain competitive. AMO science plays a substantial role in this evolution, and only with the aid of continued research and development can U.S. industries maintain a competitive edge and successfully compete in world markets. Contributions of AMO science to industrial productivity include advances in lasers in manufacturing, plasma processing of materials, and chemical manufacturing.
Lasers in Manufacturing
Lasers today find widespread application in industry to thermally treat, weld, cut, drill, mark, and trim material. One of the earliest applications, which is still
being used, is laser scribing for parts identification and inventory. Laser beams can be focused to extremely small spot sizes, allowing localized application of thermal energy. This minimizes thermal distortion in objects during cutting or welding operations and permits highly accurate micromachining (Figure 2.6). With an appropriate choice of wavelength, lasers can be used to process a wide
range of materials, including metals, ceramics, plastic, wood, and cloth, at rates that are frequently much higher than can be achieved with other methods. The ability to accurately control lasers with computers in real time makes them compatible with automated manufacturing facilities and allows for on-line design and processing changes. Although lasers are now common in industry, it is certain that their usage will increase because major improvements in the accuracy and efficiency of laser manufacturing processes can be achieved with further research to develop a better understanding of specific laser-material interactions and with the availability of lasers optimized for the particular interaction of interest. In many cases, lasers with higher reliability and lower cost are needed.
The use of lasers for microelectronic component and materials processing has advanced at a rapid rate over the past decade. Many of the important ideas and applications explored in industrial and university research laboratories have now found their way into the manufacturing environment. Other ideas are at an advanced stage of development and will become part of microelectronic technology in the near future. Integrated circuits (chips) today may comprise millions of microscopic transistors formed and interconnected on the surface of a semiconductor wafer and covering an area of approximately one square centimeter. The individual transistors and interconnects are formed through a complex sequence of growth and removal (etching) processes, many of which require submicron precision. Laser lithography systems are being developed for microelectronics manufacturing, because, in the ideal case, deposition or etching can be localized to the region where a laser impinges on the substrate, and local patterning is achieved without the use of masks or photolithographic process steps. Lasers also provide a means to repair or personalize circuits by cutting or fusing lines on chips, circuit boards, and modules. In the case of chips, this process is used to wire in new circuits or bypass broken elements.
In recent years, there has been considerable interest in the etching and ablation properties of a variety of polymers and ceramics processed in air using excimer lasers. The mechanisms that give rise to polymer ablation are complex and wavelength sensitive, but research has advanced to the point that it is now possible to utilize the ablation process of manufacturing, specifically to pattern polymer films. The primary polymer of present interest is polyimide, commonly used as the insulating dielectric on multilayered chip carriers or modules. Each layer of such a module requires thousands of blind holes typically a few tens of micrometers in diameter to interconnect metallurgy (generally copper conductors) on adjacent levels of the multilayered structure. A distinct advantage of the ablation technique for producing the blind holes is that the underlying metal layer is not damaged by the laser fluence used for ablating the polymer. For metal ablation, a much higher fluence is required. Thus, laser polymer ablation has a built-in etch-stop feature in this application, greatly simplifying manufacture. Other materials such as ferroelectrics and ceramics have also been successfully
patterned using excimer laser pulses. Here, too, the advantage is the ability to carefully control the amount of material removed per laser pulse.
Laser ablation is also important in the production of new materials. A laser is used to ablate targets containing constituents that are collected on a variety of substrates. Under appropriate conditions, high-quality films are obtained that include, for example, high-temperature superconducting oxides, optoelectronic materials, and diamondlike carbon, materials of considerable importance to microelectronic circuits and devices. One high-temperature superconducting oxide of particular interest is YBa2Cu3O7. High-quality films of this material have been grown on a variety of substrates (such as SrTiO3). The advantages of laser deposition include simplicity, stoichiometric transfer of the neutrals and ions from target to substrate independent of laser fluence, and the need for only a relatively low thermal substrate bias temperature. This lower temperature greatly reduces undesired reactions between film and substrate. Superconducting thin films have numerous applications in, for example, thin film microwave devices, Josephson edge junctions, striplines (microstrip microwave circuits), and bolometers.
Plasma Processing of Materials
Plasma processing of materials is of vital importance in many manufacturing industries (Figure 2.7), most notably the electronics industry in which plasma processing techniques are indispensable for the manufacture of very-large-scale integrated (VLSI) microelectronic circuitry. Plasma processing is also a critical technology in the aerospace, automotive, steel, biomedical, and toxic waste management industries. Plasma processing technology is being used increasingly in the emerging technologies of diamond film and superconducting film growth. The area has been reviewed recently and is discussed in a recent National Research Council (NRC) report, Plasma Processing of Materials: Scientific Opportunities and Technological Challenges (National Academy Press, Washington, D.C., 1991). The 1991 report highlights the many important industrial applications of plasma processing. These are not all described here; rather, a few representative examples are given simply to illustrate the breadth of the applications.
Plasma-controlled anisotropic etching is a critical technology used in fabricating microelectronic devices (chips). It allows pattern transfer from a developed photoresist to the underlying structure without the undercutting that is characteristic of wet chemical etching. Plasma-deposited films of silicon nitride and silicon dioxide are important for many chip applications, including passivation and the insulating of different metal layers. Plasma deposition permits the use of lower substrate temperatures than would be required with alternate techniques, allowing the material to be deposited after completion of surface features that would be damaged by a higher processing temperatures. Plasma-enhanced chemical vapor deposition is used to grow amorphous silicon films for solar cells.
Components of aircraft and automobile engines are protected against wear by plasma-spray deposition of ceramic or metal alloy coatings. Plasma-spray deposition and thermal plasma chemical vapor deposition are used to produce films of high-temperature superconductors and refractory metals. Plasma sputter deposition is important in the deposition of magnetic films for memory devices.
Despite the wide array of applications, plasma processing remains a largely empirical technology. The plasma processes in use today have been developed mostly by time-consuming, costly empirical exploration. The chemical and physical complexity of plasma-surface interactions has so far eluded the accurate numerical simulation that would enable process design. Similarly, plasma reactors have also been developed by trial and error. Nonetheless, fundamental AMO studies of surface interactions and plasma phenomena have contributed to process development by providing key insights into the operating conditions. These contributions include investigations of etching and deposition mechanisms using beams of reactive atoms, molecules, ions, electrons, and photons impinging on well-defined surfaces under controlled conditions, the development and use of diagnostic techniques such as mass spectrometry, optical actinometry,
laser-induced fluorescence, and Raman spectroscopy to measure species concentrations in a plasma, and the measurement of rate constants and cross sections for reactions that occur in technologically important discharges. However, with further basis AMO research in these areas it should be possible to develop a more comprehensive understanding of the fundamental physical and chemical reactions that take place in plasma reactors. This, coupled with the recent availability of massive computational power, should advance the design of plasma reactors from an art to a science, further enhancing their effectiveness in manufacturing.
Many AMO spectroscopic techniques, including atomic absorption spectroscopy, Fourier transform infrared spectroscopy, Raman spectrometry, and electron spin resonance spectrometry, are widely used in chemical manufacturing. Perhaps their predominant application is in the area of quality control and safety, and several representative examples are discussed here.
It is essential to protect consumers from toxic trace contaminants in either natural or manufactured products. Natural products may contain toxic elements that have been extracted from the soil and concentrated in the plant. For example, wheat selectively concentrates selenium, which is known to produce central nervous system damage at high concentrations. Although trace-level selenium is necessary for life, one can easily exceed the allowable limit. Atomic absorption spectroscopy is used to measure the selenium concentration. Traces of pesticide residues on fruits or vegetables must be eliminated, and only high-precision analytical spectroscopy (ultraviolet, visible, or infrared) can assure both distributors and users that the foods are safe. Another example of this type of problem concerns stannous fluoride (SnF2), which is used as a fluoride additive in tooth-paste. Quite often, tin ores are contaminated with arsenic or other heavy metals, and these must be reduced well below parts-per-million levels before SnF2 is used in dentifrices.
The accurate reproduction of color requires reliable, reproducible, and safe pigments and dyes. The quantitative evaluation of pigments, dyes, and other color sources requires not only a theoretical and an experimental understanding of atomic and molecular energy levels and transitions between them, but also a practical appreciation of the chemistry involved. Within the last few decades, the dangers of pigments containing lead and arsenic have been recognized. Dealing with this problem requires accurate analytical techniques. Modern photography requires a precise characterization of the dyes employed, in order to obtain a composite image that closely approximates the colors we see. The attractive colors in colored glass, synthetic rubies and sapphires, and various plastics are quantitatively characterized by use of visible-ultraviolet spectroscopic techniques allowing manufacture of reproducible products.
The pharmaceutical industry relies heavily on AMO science for the technology and instrumentation that it uses for quality control and assurance. Infrared, Raman, visible-ultraviolet, nuclear magnetic resonance, and mass spectrometry offer a broad range of approaches for analyzing the purity of samples and for characterizing their structure. The role of AMO science in precision measurement is important to this industry because many pharmaceuticals are specific isomers, and closely related molecular species are ineffective or even toxic.
New instrumentation and software have been developed in response to the increasing demands of federal and state environmental regulations. For example, inductively coupled plasma spectrometer systems have been produced that can measure 60 elements in less than one minute, using more than 5,000 emission lines to ensure interference-free analytical results. Environmental samples such as wastewater, soils, brines, and sludges can be rapidly analyzed by using such systems.
The discrete energy level structure of molecules offers the hope for selectively inducing chemical reactions using photons provided by, for example, flash lamps or tunable lasers. Selective photoexcitation of particular excited states opens the door to highly specific, mode selective chemical processing, whereby one can break a bond or lower an activation energy or make a desired reaction thermodynamically possible. As the availability of high-power lasers and flash lamps increases, it is to be expected that many applications of such synthetic chemistry will be discovered, including the laser-assisted deposition of diamond films from methane or halomethane molecules in hydrogen-rich environments.
Because of the many roles of AMO science in manufacturing, advances in the field can be expected to directly enhance economic vitality.
Information Technology, High-Performance Computing, and Communications
The success of industrial nations depends increasingly on their capability to develop and implement information technology. We live in the so-called Information Age, which is driven by the merging of communications and computing technologies. The foundations for the physical aspects of these industries lie in the AMO sciences. AMO research on laser sources and optical detectors as well as optical pulse propagation in fibers has provided the basic knowledge base for optical communications and optical data storage.
The status of and opportunities for these "photonic" technologies are discussed in the 1988 NRC report, Photonics: Maintaining Competitiveness in the Information Era (National Academy Press, Washington, D.C.). Two examples illustrate the role of AMO science in information technology—the erbium-doped fiber-optic amplifier and optical data storage.
The Erbium-Doped Fiber-Optic Amplifier
Telecommunications networks are rapidly evolving to an optical-fiber-based system that has higher performance and lower cost and requires lower maintenance than a copper-conductor-based network. Long-distance networks are now primarily fiber-optic, as are most links between local telephone switching offices. The last remaining link to the home is widely predicted to also become an optical link within the next 25 years. All of this progress is the direct result of advances in optical science, beginning with the invention of the semiconductor laser and subsequent research on fiber-optic transmission and fiber-optic systems.
One problem encountered in fiber-optic systems is that as the optical signal propagates through a fiber, it is attenuated due to losses in the fiber. To counter-act these losses, expensive optoelectronic receivers and transmitter systems are normally used to reestablish the optical signals to usable amplitudes. These costly regenerators are restricted to a particular signal format and data rate and must be replaced whenever the system is upgraded to a higher transmission rate. Recent research, however, has produced erbium-doped fiber-optic amplifiers that eliminate the need for these regenerators. Such amplifiers not only boost (by a factor of 1,000) the signal back to its original level, but are also completely transparent to signal format or data rate (Figure 2.8). Furthermore, they can simultaneously amplify several different optical signals at slightly different wavelengths at the same time. Without fiber-optic amplifiers, a separate regenerator would be required for each wavelength channel.
One remarkable feature of erbium-doped fiber-optic amplifiers is the speed at which they have progressed from the research laboratory to application. The first efficient erbium-doped fiber-optic amplifier operating in the 1.5-micrometer (µm) region important for fiber communications was reported in 1988. Within 2 years, several major telecommunications research laboratories reported impressive fiber transmission system applications, including operation at 10 gigabits per second (10 billion bits of information per second) and simultaneous amplification of multi-optical-channel signals in a single fiber amplifier system. The fiber-optic amplifier has also been shown to be suitable for amplifying optical signals with various formats, including telephone, high-speed data, and television signals. Key to practical application was the parallel development of high-power semiconductor lasers operating at the correct wavelengths to act as power sources for the fiber-optic amplifiers.
It required only 4 years for the erbium-doped fiber-optic amplifier to make the transition from research laboratory to first commercial prototype. Today, fiber-optic amplifiers are considered basic building blocks available to all telecommunication network planners, who now intend to use them in a wide range of communications systems ranging from cable television networks to transoceanic telecommunications systems. The erbium-doped amplifier illustrates clearly
that only by maintaining a broad scientific infrastructure (in this case in AMO science, laser engineering, and materials science) can a country expect to be a player in rapidly advancing technologies.
There are a number of research opportunities in this area. For example, many optical communications systems operate with laser radiation having a wavelength of 1.3 µm, which is not in the 1.5-µm operating regime of the erbium-doped fiber amplifier, and AMO research aimed at developing optical amplifiers in the 1.3-µm regime could have great impact on this installed base of systems. Another area in which AMO science can have major impact is in the application of nonlinear optical techniques to improve optical transmission in fibers. Nonlinear soliton propagation has been shown to allow optical pulses to be transmitted over very long distances without any distortion.
Optical Data Storage
With the widespread consumer acceptance of CDs for high-fidelity stereo recordings, optical information storage is now familiar to nearly everyone. This storage technology is built on focused research on low-cost semiconductor lasers and the physics of the optical interaction of light with thin film materials. It is an important technology with many applications in the computer and printing industries. Although magnetic data storage dominates the data storage market today, it is anticipated that for many applications in the future magnetic storage will be completely replaced by optical disk storage. A pocket-size optical disk can hold 300 megabytes of data. They are removable and readily transportable and can simultaneously carry mass-replicated read-only information as well as user-rewritable information.
AMO science offers the potential for dramatic improvements in optical storage technology. Information storage densities can be improved by using shorter optical wavelengths in the optical recording and reading head because shorter wavelengths can be focused to a smaller spot size. Recent advances in blue-light-emitting semiconductor lasers, in highly efficient harmonic generation of light from semiconductor lasers, and in optical parametric oscillators hold the promise of providing the required radiation. Even the limitations imposed by the optical wavelength can be overcome. Normally, optical techniques cannot resolve structures that are smaller than an optical wavelength. Recently, however, the combination of nanoscale technology with optical physics has resulted in so-called near-field optical microscopy, which can resolve structures smaller than the optical wavelength, suggesting that new very high density optical storage systems might be realized in the future.
Optical logic devices and optical computing have been elusive objectives, but they provide an area in which real progress could have a large potential payoff. The combination of optical processing with electronic processing continues to represent an opportunity to exploit the best attributes of both. Progress in optoelectronic integration is especially important for lowering the cost of optical technology for mass deployment such as is needed for the practical realization of optical interconnect technology into computing systems.
AMO science plays a pivotal role in the development and characterization of devices and materials used in information technologies. Continued research will enable further technological improvements that will keep the U.S. information industry competitive in the global marketplace.
Energy is a dominant national resource and a commanding international ''currency." The availability of energy and its management for effective and safe
use underlie the quality of life of people in all industrialized nations. The average person in the United States today enjoys energy benefits related to work, transportation, entertainment, home comforts, and conveniences well beyond those of any previous generation. To appreciate the role of energy in supporting a national position of influence and leadership, it is only necessary to recall the trauma of the "energy crisis" in the mid-seventies, to reflect on the current competition in international trade, or to recall the total necessity of various forms of energy to maintain a global military presence in a turbulent world.
Energy resources must evolve as known resources are depleted, as new technologies offer advantages of efficiency and environmental compatibility, and as expanding populations place ever-increasing demands on the quantity and quality of energy resources. This evolution of technology can proceed only by drawing on a fund of basic scientific and technical knowledge, much of which is based on the AMO sciences.
In this section, examples are presented of past and potential contributions of AMO science to the major energy technologies in use today (primarily combustion of fossil fuels supplemented by fission reactors and, to a lesser extent, by hydroelectric and solar energy), as well as those identified as possible energy sources of the future.
Combustion. Most of the energy used in the United States today is derived from organic and fossil fuels: oil, natural gas, coal, and renewables (e.g., wood). Taking account of the vast coal and oil shale resources of the nation and the technological feasibility of conversion to the currently used fuel forms, there is little doubt that organic fuels will continue to be a mainstay of the nation's energy diet for some time. Given the limited supply of fossil fuels, it is essential to learn how to use them as efficiently as possible. Because the burning of organic fuel accounts for a major part of the pollutant burden of Earth's atmosphere, it is equally necessary to learn to effectively control and limit the release of pollutants and to detect and monitor them. AMO science plays a vital role in achieving these goals.
The interiors of furnaces, combustion engines, incinerators, and other combustion reactors are hostile environments. It is difficult to put instruments into these environments without damage or destruction. But by means of laser diagnostics, it is possible to externally probe temperature, particle motion, molecular composition, and reaction rates in these hostile environments (Figure 2.9). These diagnostic methods are invaluable tools for optimizing the design and performance of such reactors.
The role of molecular physics in combustion research extends well beyond simple diagnostics. Spectroscopic investigations provide the detailed knowledge of energy levels and intramolecular processes needed to begin to understand the
course of chemical reactions. Experimental research in chemical kinetics yields quantitative information on the rates of the reactions that determine the course of combustion. Research in both kinetics and dynamics is contributing insight into these reactions at the molecular level. Elaborate models that often involve hundreds of reactive and inelastic processes make use of these data to simulate combustion. Although the modeling of chemical processes in simple laminar flames is now well established, a more ambitious goal remains. It is the development of truly predictive models of combustion that start from first principles, incorporate both molecular processes and fluid flow, and are capable of treating turbulent flames in practical configurations. The rapid increase in computational power suggests that now is the time to begin thinking seriously about what new molecular physics is required to support the development of these models. As an example, it will be necessary to know in detail the range of validity of statistical theories of chemical reaction rates, which entails understanding how quickly energy moves through molecules and whether the end result is random. This, in turn, requires breakthrough research in such areas as the use of femtosecond laser techniques to probe the evolution of highly vibrationally excited molecules. Many such examples exist; the potential future contributions of optical and molecular physics to optimizing combustion processes are great indeed.
Fission. The importance to the nation of maintaining a strong program in AMO science was illustrated during the energy crisis in the mid-seventies, when it appeared that there would be a shortfall in the nation's ability to keep up with the demand for reactor-grade uranium. AMO scientists were quickly mobilized, and two methods of laser isotope separation, one involving atoms and the other molecules, underwent intensive investigation. New techniques were devised for precision spectroscopy and for interpreting complex spectra, and laser systems were developed that could be precisely tuned to the required frequencies. The decrease in demand growth in the eighties converted the original objective to a more difficult one, namely, to enrich uranium at the world's lowest cost in order to maintain the U.S. share of the multibillion dollar enrichment market. The atomic vapor process was chosen for further engineering development at Lawrence Livermore National Laboratory. This program uses the world's most powerful visible laser system and has produced substantial quantities of reactor-grade uranium. This laser-based technology is on track toward commercial deployment as the existing gaseous diffusion plants are retired.
Solar Energy. The energy flux from the sun at Earth's surface amounts to about 1 kilowatt per square meter (kW m-2). It is a generally clean and nonpolluting energy form and is attractive from many points of view. Many different approaches can be taken to harnessing solar energy, including direct conversion into electricity via solar cells, natural and artificial photosynthesis as occurs in plants, and solar heating and cooling of buildings. Solar energy also drives surface winds and ocean thermal currents that can be tapped as energy sources.
Even the proposed use of hydrogen for transportation fuel is in some measure solar if ocean thermal currents or other solar collectors are used to extract hydrogen from water.
The key to using solar energy for electrical power lies in being able to efficiently convert it to electricity and to store it for times when the sun is down. The technologies for both exist, and it is now a matter of making these economically competitive. The production of large-area amorphous silicon photocells by plasma deposition (see the section above on plasma processing) is one promising approach.
Photosynthesis is the way that nature converts solar energy into organic fuel. AMO scientists have been gaining insights into the interactions of light and matter and the molecular processes that occur in this conversion process. Some successful attempts at artificial photosynthesis have now been recorded, and both natural and artificial photosynthesis figure into the energy equation.
Fusion: Magnetic Confinement. Fusion, as it is envisioned, is a relatively benign energy technology in terms of its impact on the environment and threat to health and safety of people. The technology, however, is difficult and complex, and 40 years of research have just been rewarded with the demonstration of a "break-even burn," in which as much energy was produced through fusion as was used to create the burn. Thus, fusion is not yet a viable and economic source of energy. Further research in this area is merited, however, because the rewards of success will be great given an essentially limitless supply of fuel in the oceans.
Feasibility depends, among other things, on characterization and control of energy loss mechanisms in the reaction vessel, which can arise from interactions between electrons or photons and ions of heavy elements liberated from the reaction vessel by the various forms of radiation present. These energy loss mechanisms require new knowledge of the spectroscopy and excitation-recombination cross sections of atomic ions and information about the effects of strong electromagnetic fields on atomic properties and processes.
A valuable contribution of atomic theory to the design of magnetic fusion reactors has been the demonstration that the presence of small fractions of highly charged heavy ions can lead to significant energy loss. This has led to the replacement of heavy metals such as iron and molybdenum in the reactor walls, divertors, and delimiters by lighter materials such as graphite. Even with such low-z atoms, it is important to minimize wall erosion and the transport of such impurities into the central plasma region. Studies of plasma-edge effects have been recognized as being of paramount importance and are a significant component of the International Thermonuclear Experimental Reactor (ITER; Figure 2.10).
Improvements in the techniques of plasma spectroscopy have led to more detailed information on temporal and spatial variations in plasma properties such as electron density and temperature, the strength of magnetic fields, and the rate
of flow of particles in or out of the plasma. Studies of the intensity and line profiles of the radiation emitted by the plasma provide detailed information concerning the impurity ions as well as dominant electrons and hydrogenic species present in the plasma core. Further data can be obtained by studies of the radiation induced by the injection of laser or particle beams. Studies of Thompson scattering of laser light and the photons emitted following electron capture by impurity ions from neutral hydrogen beams have been particularly valuable in this respect.
Fusion: Inertial Confinement. The effort to achieve laboratory ignition of thermonuclear fuel by laser implosion of small pellets rests on a long history of AMO science advances that contributed to the understanding of dense plasmas and also led to the development and construction of the powerful laser sources needed to make the plasmas. In the case of lasers, these contributions range
from the understanding of ionic spectra in solid-state laser materials and of excimer laser systems, to the linear, nonlinear, and electro-optical techniques developed for beam manipulation and control. X-ray and laser plasma diagnostics also play an important role in evaluating target performance. In addition, the contributions of atomic physics to the understanding of interactions in plasmas of highly stripped atoms—particularly for advanced targets employing complex mixtures of elements to control energy transport and hydrodynamics during the implosion—are important not only for laser-driven targets, but also for heavy ion drivers, which are viewed as attractive candidates for commercial energy production. Correlation of theoretical models with experimental data from existing laser target facilities, or from the next-generation laser driver proposed for reaching thermonuclear ignition, will be crucial to evaluating the feasibility of inertial confinement fusion for energy production as experiments push into regimes of higher plasma density than have been explored to date.
Efficient Use of Energy
Efficient use of energy is an extremely important part of any energy policy. Prior to the energy crisis of the mid-seventies, the nation had become extremely cavalier about energy use, and waste was rampant. Since that time, people have become more educated and caring about conservation, and this increased awareness has reaped rewards in lower energy consumption.
Conservation in lighting has been strongly affected by AMO science. The fluorescent lamp and arc lamp, widely used for lighting in businesses, homes, factories, and streets, function by the excitation and fluorescence of atoms and molecules in the gas phase, accompanied in some cases by conversion of ultraviolet light to visible at a phosphor on the walls. These lamps were initially developed without a detailed understanding of much of the physics involved, but now detailed gas-phase models and an understanding of gaseous electronics are used, together with observations, to improve and alter their properties. This progress toward a comprehensive understanding of lighting is a triumph of plasma physics combined with AMO collisional radiative theory and experiment. The biggest challenge for the future is to increase phosphor lifetime by reducing surface damage by particle bombardment, requiring further studies of particle-surface interactions.
Achieving harmony between energy production and use and a healthy environment is absolutely essential. However, the atmosphere is heavily burdened with carbon dioxide, a serious concern in terms of global change, and with the oxides of nitrogen and sulfur as a result of organic fuel consumption. With continued research and development, AMO science can reduce the environmental impact of energy production using fossil fuels
through improved understanding of combustion reactions and combustion reactors and through better diagnostic procedures to detect and quantify pollutant emissions. AMO science is also central to the development of less environmentally harmful sources of energy such as solar and fusion power.
Spiraling energy use and other human activities have led to measurable effects on the global environment. There is increasing international concern about the impact on Earth of human activities, in particular in the areas of global warming, depletion of the ozone layer, and pollution problems such as acid rain (Figure 2.11). As discussed in the section on atmospheric and environmental science, AMO science plays a pivotal role in the development of computer models to analyze the effects of changes in atmospheric composition and in the provision of reaction rates, collision cross sections, line shapes, and so on, for inclusion in such models. AMO science also contributes to reducing the load on the environment from energy use. Monitoring atmospheric constituents, modeling the impact of introducing foreign gases into the atmosphere, and predicting the eventual consequences of changes in atmospheric constituents are important challenges to AMO science. Global change considerations can also stimulate experimental and theoretical advances in AMO research. For instance, recent appreciation of the role of heterogeneous processes in the generation of acid rain and the destruction of stratospheric ozone has led to development of a number of novel experimental techniques to study mass accommodation and heterogeneous reaction processes on aqueous and aqueous-acid liquid and ice surfaces. The data obtained by these methods have led to a novel model of the mass accommodation of gaseous species onto liquid water surfaces.
The most difficult atmospheric challenge is to reliably quantify a wide variety of trace species, in both gaseous and condensed phases, which may be important to any given environmental problem. Most of the atmosphere is relatively nonreactive nitrogen, oxygen, and argon. The concern is the minor constituents of the atmosphere, that is, gaseous species that may constitute less than 1 part per trillion by volume (pptV) and condensed cloud droplets and aerosol particles that occupy a fractional volume of about 1 part in 10 million. Reliable quantification of important gaseous trace chemical species varies in difficulty. Carbon dioxide at about 355 parts per million by volume (ppmV) is relatively easy to measure, while the hydroxy radical at 0.5 to 0.005 pptV during daylight poses a much more difficult problem. Most other trace species of interest fall between these two extremes.
Until relatively recently, most atmospheric trace species were measured by
grab sampling followed by batch analysis using either wet chemical or chromatographic techniques. However, advances in electro-optics (especially tunable lasers), microprocessor-based control electronics, and other technologies have recently spurred the development of highly sensitive, real-time spectroscopy-based measurement techniques for many key atmospheric trace species.
The sensitive, real-time trace species measurements made possible by laser-based and other advanced electro-optical techniques do more than quantify the current chemical content of the atmosphere. Simultaneous measurement of interrelated species with good time resolution allows detailed tests of the photochemical mechanisms embedded in atmospheric models. Furthermore, coupled with micrometeorological measurements they allow direct determination of key trace gas fluxes between the atmosphere and Earth's surface, permitting experimental confirmation of critical atmospheric model boundary conditions.
Atmospheric laser-radar (LIDAR), a form of remote laser spectroscopy, provides a graphic example of the use of AMO science in field measurements and is the oldest field application of laser spectroscopy in atmospheric remote sensing. In its simplest form, fluorescence LIDAR, a laser is tuned to a resonance excitation of the species of interest. The laser is projected into space, and the fluorescence from excited species is collected and detected by a telescope at the laser source. By collecting the signal as a function of time, the density as a function of distance can be measured. For example, even in the early seventies, investigators were detecting sodium atoms in the mesospheric layer 80 to 100 km above Earth's surface. This region is too high for balloons and too low for satellites, and previously could be accessed only by rockets.
LIDAR provides the opportunity to study conditions on a continuous basis. Mesospheric LIDAR has profited immensely by the development of stable, precise laser sources. By using lasers that are locked in absolute frequency with megahertz accuracy, observers are able to remotely measure not only densities but also wind velocities with an accuracy of 3 m sec-1. In addition, by accurately recording the profile of the excitation lines and deducing the population of hyperfine levels of the ground state, the temperature of the mesosphere can be measured.
The mesosphere is an area of fundamental importance to atmospheric physics. The measurement of winds and densities in the layer allows observation of atmospheric gravity waves reflecting the dynamics of the atmosphere. In addition, mesospheric measurements are of importance to both global warming and ozone depletion studies. Carbon dioxide and methane, which lead to warming of the troposphere (the greenhouse effect), lead at the same time to cooling of the top of the atmosphere including the mesosphere. It is expected that over the next century, while the troposphere warms a few degrees, the mesosphere will cool by as much as 20° Celsius. Already there have been reports of a steady lowering of the mesospheric boundary consistent with a cooling of that layer. This may be an early measure of a basic change in the atmosphere.
The sodium (Na) atoms in the mesosphere come primarily from meteoric dust. The sodium atoms react to form NaO, NaO2, and NaOH, which migrate into lower regions of the atmosphere. There, reactions with hydrogen chloride (HCl) result in the release of chlorine (Cl) as a free atom, which them enters the catalytic ozone destruction cycle. It is now thought that mesospheric sodium
may be a significant contributor to the complex balance of processes that determines the rate of destruction of ozone in the upper atmosphere.
LIDAR techniques have also been used in many other remote sensing applications, particularly for monitoring pollution sources. For instance, LIDAR has been used to monitor the emission of sulfur dioxide and other gases from power plant smokestacks; it allows off-site monitoring and precise pinpointing of sources. Law enforcement officials have expressed an interest in the use of LIDAR, for example, to detect effluents characteristically associated with clandestine drug operations, and there is interest in extending the techniques to enforce nuclear nonproliferation, as well as to detect biological and chemical warfare violations. An area of major potential impact is the use of remote sensing devices for monitoring the evolution of gases in nuclear waste storage depots, where direct access is limited. In all these areas, LIDAR promises to serve as one of a family of tools brought to bear on problems of immense concern to the nation.
Remote sensing of atmospheric constituents and atmospheric pollutants using LIDAR and other techniques is a critical part of the global change research effort. Advanced field systems for ground-based monitoring and advanced in situ detection systems for finding trace and unstable species are urgently needed and are dependent on the availability of stable, tunable, field-qualified laser sources of sufficient power and energy output. Problems in local and regional air pollution, lake, river, and marine pollution, groundwater contamination, and toxic waste disposal all pose similar modeling, field measurement, and laboratory and theoretical challenges, and all benefit from AMO science. Even ecological problems with a large biospheric component and environmental health issues with a major focus on human physiology still require sophisticated chemical and physical measurements. Determining the relevant interactions of atoms, molecules, aerosols, and sunlight is a great challenge to AMO science, as is the task of developing computer models that will predict the effects of pollutants.
AMO science has been, and will undoubtedly continue to be, important in national defense. Lasers and electro-optics have come to the forefront in a variety of ways in modern warfare. In addition, atomic and molecular processes and spectroscopy play an important role in such areas as radio propagation in the ionosphere, atmospheric infrared transmission, infrared background emissions, radiation trapping in nuclear bursts, nuclear blackout of electromagnetic propagation, and missile plume detection and analysis.
Significant among the early applications of lasers were rangefinders and target designators; these applications were based on the ability of the laser to produce short pulses of radiation and to be pointed precisely at distant targets. These early applications have expanded to include laser systems for optical countermeasures, gyroscopes, range-resolved Doppler imaging radars, illuminators, space communication systems, fiber-guided missiles, and fiber sensors. In the area of C3 (command, communication, and control), the Department of Defense (DOD) has both contributed to and benefited considerably from fiber-optic communications technology. In recent years, research programs have been under way to apply lasers to optical interconnects and optical information processing. It is noteworthy that, in all of these areas, a good deal of the laser technology and applications development that has been carried out with DOD funding has proved useful in nonmilitary areas.
Weapons Systems and Delivery
Laser rangefinders are used to measure distances for ground-based munitions; Q-switched Nd: YAG solid-state lasers producing pulses of the order of 5 to 10 nanoseconds are capable of range resolution of the order of 1 meter. The new generation of rangefinders will use semiconductor laser diodes or diode-based eye-safe solid-state lasers.
Target designators employ laser beams reflected from a distant target to guide a bomb or missile containing a smart seeker onto the target. These systems have proved important in the precision guidance of munitions. They were used successfully in the Vietnam War and in Desert Storm to avoid collateral damage. Lasers have also been used to illuminate distant targets for optical surveillance and reconnaissance.
Atomic opacity is an important problem in nuclear weapons physics. There have been few experimental measurements of the opacity, that is, the degree of absorption of transmitted light, for plasmas at moderate or high temperatures and density. The calculation of opacities requires a large atomic or molecular database, with information on the energy levels of several ionic stages of each species in the plasma and of the corresponding radiative transition probabilities and line shapes. Available results are not of sufficient accuracy for current needs, especially for many-electron atoms, and do not take advantage of the large increases in computer power and the advances in theoretical techniques achieved in recent years.
Two efforts are under way to provide better opacity data. One project, carried out as a collaboration between the United Kingdom and the United States, exploits the configuration-interaction codes for atomic structure and matrix codes for continuum wave functions. Although this effort has so far produced few new opacity data, the physics underlying these codes is excellent, and valuable intermediate results have been obtained, such as collision cross sections, energy levels,
and transition probabilities. Another approach has been the calculation of atomic wave functions using a model potential. The first results of this technique have resolved several outstanding problems in astrophysics, and the method is being evaluated currently for the high-temperature plasmas appropriate to fusion and defense applications.
AMO instrumentation is crucial to several defense programs for remote sensing. This includes laser-radar (LIDAR) monitors for chemical and biological warfare agents and fiber-optic sensors. Pulsed Doppler laser radars using heterodyne detection have been successful in providing complex, time-resolved images of distant scenes in the six-dimensional space of position and motion. The utility of laser radars for terrain-following guidance and obstacle avoidance for low-flying aircraft has been demonstrated. Replacing microwave radars, these LIDARs allow aircraft to avoid detection.
Lasers tuned to specific infrared wavelengths provide effective monitoring systems for chemical and biological warfare. Fear of the use of such warfare techniques on embattled populations can be reduced with effective remote monitoring techniques, using the concepts of AMO science. Further discussion of this topic is found in the section on global change. Fiber-optic sensors have proved particularly effective as acoustic sensors, providing much more accurate undersea measurements than traditional sonars. Fiber sensors can remotely and/or passively detect motion, strain, temperature changes, and magnetic fields and can be coupled to optical detectors. These versatile sensors can be used in many places, from the battlefield to embedded in the skins of aircraft.
Atmospheric transparency is important in the design and operation of infrared systems such as the forward-looking infrared system (FLIRS), heat-seeking missiles, and infrared lasers. Data on atomic and molecular processes are vital to the understanding of atmospheric and meteorological phenomena that affect military scenarios. Much effort has gone into developing both highly resolved (HITRAN) and broadband (LOWTRAN) transmittance models. This has involved the measurement and calculation of line positions, lower-level energies, line strengths, and pressure broadening and shifting coefficients for water vapor, carbon dioxide, and other absorbing molecules occurring in significant quantity in the atmosphere.
Infrared signatures of missiles are being examined through spectroscopic analysis of rocket motor plumes including measurements of rotational temperature and features such as the blue spike in the 4.5-mm carbon dioxide band. In recent years, these techniques have been extended to monitoring the emissions accompanying a missile launch, fuel spill, ground test, and manufacturing process. In the case of a missile launch, the monitoring activity is combined with mobile tracking and computer modeling to predict the shape, size, and concentration
of gases in a plume as a function of local conditions such as meteorology and terrain. One goal is to determine where or when certain operations should not be carried out because of possible exposure hazards to local populations.
Countermeasures on the electro-optical battlefield are required to defeat commonly used infrared and near-infrared sensor and imaging systems. Effective laser countermeasures to such optical systems are currently under extensive development. Defeating visible and near-infrared imaging systems and missile heat seekers is the major thrust of these efforts. Because of the intensity gain due to focusing and imaging in optical systems, countermeasure systems can use relatively low power lasers. Infrared heat-seeking missiles (SAMs) represent a serious threat to both civilian and military aircraft that operate in or near areas of strife. When used against civil aviation, they can become a weapon of terror. Currently, military aircraft are equipped with missile countermeasure systems that are successful against early generations of heat seekers. However, newer heat-seeker systems have been designed to counter these countermeasures. It is a challenge to the countermeasure designer to develop new systems that do not have the shortcomings of those is current use. A laser can be used either to permanently damage or to temporarily dazzle sensors, rendering them ineffective. This technique can be used to take out of commission visible and near-infrared systems, which are used in surveillance, reconnaissance, discrimination, weapons direction, and guidance. Typical sensors that can be blinded are the human eye, charged-coupled device (CCD) focal plane arrays, and image intensifiers that require lasers in the 0.4- to 1.0-mm range.
C3—Communication, Command, and Control
Fiber optics are being used increasingly in military communication, command, and control systems because of their high bandwidth, small size, freedom from noise and cross talk, ability to withstand electromagnetic impulses, and the security provided by their low radiant emission. They interconnect computer systems within command centers, within aircraft and within ships. With the increasing ability of computers to handle the vast amounts of information that electro-optical systems make available, the high bandwidth of laser-driven fiber-optic communications becomes crucial. This makes possible increased ''fusion" of the data from many sensors into intelligible, readily used signals to improve battlefield response. Because fibers are so small and flexible, they can be used effectively to guide missiles, allowing them to be remotely maneuvered in real time; such systems have been deployed and are in the military inventory.
The use of fiber-optic communications for interconnecting computers is well established. Interconnects within computers on a subsystem level are under
development, and board-to-board and chip-to-chip optical communications are active research areas. These interconnects will move down into smaller subsystem levels as more efficient laser sources are developed. Microcavity lasers will perhaps be used here. Interconnect research is driven by the need for teraflop (1012 floating point operations per second)-level computation power for automatic target recognition, antisubmarine warfare, electronic warfare, and electronic intelligence, together with the fact that interconnect bottlenecks limit the development of teraflop systems using silicon technology. The high degree of parallelism, as well as the speed of optics, also points to the potential for the direct use of optics in processing.
The infrared, visible, ultraviolet, and X-ray background of the atmosphere and its variability with time and location must be accurately delineated to help in modeling the operation of sensors and communication systems. Here also, a vast collection of spectroscopic data is needed, and considerable effort is required to understand the effects that high-altitude atomic and molecular dynamics have on this background.
Disturbances in the atmosphere due to nuclear explosions can cause major changes in its ionization and thermal and chemical structure. The increase in ionization leads to absorption of radio waves, and a radio blackout occurs, in which communications are disrupted. The disturbed atmosphere is a copious emitter of radiation, which can serve as a remote diagnostic probe of the changing environment. The atmosphere, as it responds, can be regarded as a dynamically evolving laboratory of AMO science. The description of its evolution and its return to the undisturbed state depends on a complex variety of atomic and molecular processes involving ions, neutral atoms, and charged and neutral molecules in abnormal distributions of energy levels. With the atmospheric test ban and the moratorium on underground testing, laboratory simulations of the highly disturbed atmosphere are necessary.
Navigation systems for the military increasingly use technologies having AMO science as their base, including gyroscopes and atomic clocks. The laser gyroscope has both civil and military applications in guidance and navigation. Active and passive laser gyros have been shown to be useful in applications that require high rate and/or high stability; being nonmechanical they are not subject to wear and tear. All-solid-state fiber gyros are expected to provide inexpensive replacements for mechanical gyros in a number of guidance applications.
The cesium atomic clock is used on the Global Positioning System, which provides information on positioning that is vital for military commanders. This technology is rapidly becoming available to consumers. Future ideas for atomic clocks include solid-state silicon-based microcavities. These clocks need to be reliable and inexpensive.
National defense and security are dependent on contributions from AMO science and will continue to be as new optical technologies
are introduced. Further, simulation of warfare scenarios is becoming increasingly important in a downsized military establishment, and there is a clear need for a wide variety of accurate atomic and molecular data that describe normal and disturbed atmospheres and for variability models to simulate sensors and other electro-optical systems.
Health and Medical Technology
Lasers. Lasers have become indispensable tools in numerous therapeutic and diagnostic medical procedures. A laser beam can be precisely focused at discrete lesions in the eye, the skin, or other exposed areas and can also be directed through thin flexible optical fibers, allowing, in principle, laser treatment of any body organ through a minimally invasive procedure (Figure 2.12). Today lasers
are used to treat a wide variety of medical problems and frequently provide an attractive alternative to surgical intervention. With further research into treatment methods and development of low-cost laser systems tailored to medical needs, it is clear that lasers will become even more important in medicine in the future.
The use of lasers in medicine and surgery has expanded rapidly for two reasons. First, a broad range of lasers have become available, allowing matching of the wavelength and temporal characteristics of the radiation to each particular clinical problem. Second, there has been considerable progress in understanding laser-tissue interactions. Laser energy deposition results in localized heating that can, for example, cause blood vessels to coagulate or lead to tissue ablation. At very high pulsed laser intensities, optical breakdown occurs, resulting in plasma formation and almost complete absorption of the laser pulse in a tiny volume.
Research is under way using low-intensity laser radiation to detect and treat cancer. Tumors tend to collect and store certain body pigments such as porphyrins, and cancerous tumors will take up and retain exogenously administered pigments and certain fluorescent dyes. Thus an early-stage cancerous mass can be detected through its concentrated fluorescence under blue or near-ultraviolet laser irradiation during, say, a fiber-optic catheter examination. A similar procedure can also be used to selectively kill cancerous cells. Illumination of porphyrin molecules with red light can result in the production of singlet oxygen, which is highly toxic. Thus, porphyrin-laden cancer cells can be destroyed by irradiation with red laser light, whereas interspaced normal cells are essentially unaffected. Another laser application in cancer therapy is laserthermia. Cancerous tumors are poorly supplied by blood vessels compared to normal tissues and cannot readily disperse heat. Thus, it is possible to locally heat and destroy a tumor using laser radiation introduced by an optical fiber.
Lasers are finding numerous applications in ophthalmology. Photocoagulation using an argon ion laser is now the standard treatment for diabetic retinopathy, although diode lasers are also now being considered for this application. Various techniques involving small laser perforations are being used to treat glaucoma. A common problem following cataract surgery is that the membrane holding the implanted intraocular lens becomes opaque. Optical breakdown induced by a Q-switched Nd:YAG laser is used to cut the membrane, removing it from the optical path. Studies of the cutting process have revealed a complicated series of events initiated by laser-induced breakdown, which can cause cavitation, shock waves, and liquid jets and create unwanted damage. These studies have led to an understanding of the scaling laws for such damage and have prompted investigation of the use of less energetic picosecond pulses for initiating optical breakdown. Picosecond pulse microsurgery has application in cutting unwanted structures near the retina, which would be damaged if nanosecond pulses were used. Lasers are also used in correcting vision defects, most of which are due to small anomalies in corneal curvature (the curved corneal-air
interface provides ~70% of the total refractive power of the eye). Research is under way using argon fluoride (ArF) excimer lasers to reshape imperfect corneas through selective ablation. In addition to reducing the need for eyeglasses, this technique can also be used to remove corneal scars.
Lasers are also used as a scalpel (Figure 2.13). If the laser parameters and focusing are chosen such that the beam is intense enough to ablate and cut through the material directly in its path, and provide sufficient heating of neighboring tissue to cause coagulation, incisions can be made with minimal bleeding, which is especially valuable for surgery on vascular organs. If a short-pulse (=1 msec) laser is employed that is strongly absorbed in the target, the absorbing volume ablates before there is time for appreciable outflow of heat, and consequently each pulse precisely removes a thin layer of material. For example, lasers have had a dramatic positive impact on the treatment of gynecological diseases in women. The precise application of intense laser energy permits destruction of only damaged tissue while maximizing preservation of the normal reproductive tract. A number of new laser technologies are finding their way into clinical use. The pulsed Er: YAG laser (with a wavelength of 2.9 µm) has been shown to precisely cut both bone and soft tissue. The pulsed Ho: YAG laser (with a wavelength of 2.1 µm) is not as strongly absorbed by tissue water, resulting
in greater residual tissue damage, but has the advantage that it can be transmitted by conventional quartz fibers, allowing its use in procedures requiring fiber-optic delivery of laser light. Thermal and acoustical effects of pulsed lasers are being actively modeled, and deleterious effects, such as the generation of gas bubbles, are being studied. Diode laser systems are also now reaching output power levels suitable for use in tissue cutting and welding.
Both Ho:YAG and xenon chloride (XeCl) lasers have been used in clinical trials as an alternative to balloon angioplasty. In laser angioplasty, pulsed-laser ablation is used to clear obstructing arterial plaque. However, this requires precise removal of as much diseased tissue as possible without puncturing the vessel wall. Diagnostic feedback methods such a ultrasonography and tissue spectroscopy are being studied for this role. The acoustic shock waves that result from pulsed-laser ablation are used to clinical advantage in the treatment of kidney stones. Stones lodged in the urinary tract are often fragmented by shock waves when illuminated with visible pulsed dye laser light. Such laser procedures provide an attractive alternative to surgery.
Medical Imaging. Medical imaging frequently employs X-rays with the attendant risk of cancer induction. As an alternative, researchers are exploring the possibility of imaging using light. This is a difficult problem because strong multiple scattering occurs in tissue, destroying the image contrast and making it difficult to extract spatially resolved information from measurements of the transmitted or backscattered radiation. However, multiply scattered photons travel a greater distance in the target before emerging than do those transmitted directly, and recent work has demonstrated that illumination with picosecond laser pulses, in combination with the use of an ultrafast optical gate, provides a means to selectively discriminate against multiply scattered light. Numerous variations of this simple idea are currently being explored. Time-gating has been used for optical transillumination of tissue, and the images obtained have been investigated for applications such as the early detection of excess blood in the brain (detection of strokes) using multiple source-detector pairs to locate the source of absorption by blood. Knowledge of the absorption spectrum of blood can be used to guide the selection of wavelengths optimal for detection of oxyhemoglobin or deoxyhemoglobin. Picosecond lasers are also being used in the early diagnosis of breast cancer. Initial imaging experiments used mode-locked dye lasers as sources and streak cameras or fast microchannel devices as detectors. Such time-domain imaging involves expensive lasers and detectors and is quite complex. As a consequence, a number of laboratories are now investigating the use of simpler frequency-domain techniques in imaging.
Femtosecond laser pulses have been used to obtain optical echoes from structures within the eye and from different layers in skin. Such optical ranging has demonstrated better spatial resolution than is currently available using ultrasound techniques; however, the need for ultrafast pulsed lasers and gated detection
limits the clinical utility of the technique. Recent work has shown that ranging measurements can be made by using a continuous wave low-coherence-length superluminsescent diode in an interferometer system. Measurements have been made on eyes and arteries in vitro, and a number of ophthalmic applications, such as early detection of damage to the optic nerve due to glaucoma, are being investigated. Femtosecond pulse technology is also being used to study the rapid processes involved in vision.
Cell Manipulation. Optical trapping (see Chapter 1) has a myriad of potential research and therapeutic uses in medicine, and commercial optical trapping systems have recently become available. Specifically, optical traps are now being used to study the forces involved in the locomotion of biological macromolecules and to manipulate and position cells ("optical tweezers"). Cells can be held for fusion or perforation with a second laser to facilitate genetic manipulation or in vitro fertilization.
Monitors. Measurements involving isotopic tracers are widely used in both clinical medicine and basic research. For example, this approach is being used to monitor the metabolism of calcium in the body, a process of interest in many areas, ranging from the study of osteoporosis to the feasibility of long-term spaceflight in zero gravity. Isotopically labeled calcium is consumed in the form of milk or other foods and subsequently measured in bones or blood. Traditional techniques that make use of radioactive tracers are clearly not desirable, especially in studies involving growing children and pregnant women. A better approach is to use a stable calcium isotope, such as 48Ca. The 48Ca atoms present in a blood or bone sample can be monitored with high sensitivity using high-resolution mass spectroscopy or laser spectroscopy. (These spectroscopic techniques can also be used to monitor heavy metals and other species present in the blood.) Studies using isotopic tracers require the availability of large amounts of isotopically enriched material, but these can be obtained by using laser isotope separation.
Radiation and Health Physics
Health physics deals with understanding the interaction of ionizing radiation such as alpha particles, beta rays, gamma rays, and neutrons with living systems and with the design of instrumentation to measure sources of radiation and estimate possible harmful effects on living systems. Such studies rely heavily on AMO science and are vital to human society because ionizing radiation occurs in the environment in the form of natural radiation and cosmic rays and also because it is introduced by human activities such as medical and industrial uses of radiation and nuclear energy technology.
Once energy is deposited in living tissue by ionizing radiation, a complex sequence of events occurs starting at the atomic level, continuing through a
chemical phase, and ending with the observed biological and medical effects. Microscopically, collisions of energetic particles with atoms and molecules result in the production of excited atoms, ions, and secondary particles, most importantly electrons. Analyzing the subsequent chain of events and their possible effects requires a detailed spectroscopic knowledge of the pertinent excited and ionized states and the cross sections for all the major collision processes operative, remembering that the predominant chemical reactions in ground and excited-state collisions may be different. Further research is required to establish a more complete database of cross sections for collisions of electrons, protons, and heavy ions with molecules, especially polyatomic molecules of biological importance such as water and the hydrocarbons.
AMO science is also essential in the design of dosimeters to measure radiation fields and sources. Because many such instruments are dependent on excitation or ionization of some medium, research on the interaction of radiation with matter is required to calibrate and interpret the readings of such instruments. One outgrowth of dosimeter research was the development of resonance ionization spectroscopy, which can provide isotopically selective trace element detection and is now used to monitor accidental emission of long-lived radio isotopes and other species.
Design of Bioactive Molecules (Pharmaceuticals)
Molecular theory is making numerous contributions to the design of bioactive molecules, including drugs for treatment of disease, and herbicides and pesticides for agricultural use. The principal "tools" of molecular theory encompass quantum chemical techniques and a variety of methods that make use of empirical potential-energy functions, such as molecular dynamics and Monte Carlo simulations. The goal of most applications of these methods in the design of bioactive molecules is a determination of the geometric and electronic structural properties of the molecules of interest. Two overriding concerns in this regard are the electronic characteristics (e.g., charge distribution, dipole and quadrupole moment, and molecular electrostatic potential) and the nature of the bioactive conformation(s) of a molecule. Quantum chemical methods are playing an important role in dealing with these concerns for a wide variety of small molecules of biological interest. Due, however, to the significant number of torsional degrees of freedom possessed by a substantial number of these molecules, it is necessary in many cases to employ a "hybrid" procedure, which uses empirical potential-energy-function-based conformational searches followed by some form of quantum chemical calculation to determine the required electronic properties of appropriate conformers. In cases in which suitable potential-energy functions are unavailable, semiempirical quantum chemical methods have been used, but, as noted above, these methods are restricted in the size and conformational complexity of the molecules that can be treated.
The development of transferable potential-energy functions, coupled with the rapid growth in computational power of today's high-performance computers, has extended the range of problems that can be treated by molecular theory methods to include bio-macromolecular systems such as proteins and nucleic acids. Of especial interest here are a number of molecular dynamics and Monte Carlo-based techniques (e.g., thermodynamic integration and perturbation methods) for calculating thermodynamic properties such as the free energy of ligand-protein binding—a property that is notoriously difficult to calculate. These techniques have also been employed in studies of, for example, the solvation and conformational free energies of small molecules in aqueous and other solvents.
Advances in ab initio electronic structure theory in combination with new computing capabilities now permit approximate but realistic calculations on sizable molecules. In particular, such calculations can guide the synthesis of new drugs and can suggest strategies for creating new bioactive molecules. However, improved quantum mechanical methods for treating larger systems by semiempirical or ab initio techniques are required, together with development of improved parallel molecular dynamics algorithms to allow for more realistic simulations and for treatment of supramolecular systems such as biological membranes.
AMO science is important in a number of health-related areas, especially medicine. With the development of new low-cost lasers and delivery systems, laser-based procedures will become more widely accessible in the future. Also, new optical techniques promise to reduce the reliance on X-rays in medical imaging. Molecular theory is making significant contributions to drug design.
The U.S. space program has sizable efforts in astrophysics, space science, and atmospheric and environmental science as well as in a number of other areas, and the impact of AMO science in these areas has already been emphasized. In addition, AMO science, particularly through lasers, plays a strong role in space technology.
Measurement and Sensing
Until a few years ago, virtually all sensing from space was done passively. Active remote sensing systems using space-based lasers (LIDAR) are expected to play an increasingly important role, providing information for meteorology and pollution monitoring as well as long-term studies of global climate change.
LIDAR is a form of optical probing by light scattering that can be used to measure the total aerosol distribution and the concentration profiles of aerosols and trace gases, as well as wind vectors in the atmosphere. For the past several years, NASA has been developing the Laser Atmospheric Wind Sounder (LAWS) LIDAR system for the worldwide mapping of winds from space. Another program, LIDAR In-space Technology Experiment (LITE), is aimed at range-resolved studies of cloud top heights, a parameter important in weather forecasting. LITE II, a follow-on system, is being designed using a tunable Ti:sapphire laser for vertical profiling of the atmospheric temperature and water vapor content.
Lasers are used to accurately measure and control baselines in coherent interferometry, to obtain high angular resolution in the microwave region. These uses may be for astronomical applications or for tracking satellites at great distance. Research is under way to connect several microwave antennas with lasers by means of fiber-optic systems, but achieving this goal requires a detailed understanding of the noise properties of semiconductor lasers modulated by ultrastable microwave sources and the effects of fiber-optic transmission.
Spacecraft Navigation and Communication
Satellite-based laser communication links are being explored because they can be made very directional. This capability offers security, of considerable military significance, and the potential for very long distance communications, as needed for the deep-space network. Laser systems are projected to require lower power, weight, and volume than comparable microwave links. The military plans to fly diode-pumped Nd:YAG lasers in the first generation satellite-to-satellite communications technology, but these should ultimately be superseded by semiconductor diode lasers. The challenge in satellite communications is to achieve ultrareliability over long periods not only in laser performance but also in pointing and tracking. The resolution of these technological issues promises spinoffs into commercial applications. For example, diode-pumped solid-state lasers can provide ultrareliable sources with a wide array of uses in manufacturing and medicine.
The introduction of deep-space satellite missions using radio-frequency signals for communication was the impetus for a field known as radio science, in which the radio communication signals are used to extract scientific information about the intervening medium. The development of communication systems using optical signals enables an entirely analogous field of "light science." This field includes the study of planetary atmospheric absorption at optical wavelengths, fine-scale scattering from planetary ring systems, and integrated forward scattering over interplanetary distances in the solar system dust field. Furthermore, many previous radio-frequency experiments that were contaminated by charged particle density fluctuations from the solar wind, like gravitational body bending of electromagnetic waves, can now be performed without those
disturbances, because the effects of charged particle fluctuations fall off as the square of the carrier frequency.
The development of solar-pumped lasers is being pursued for space power transmission and propulsion. New potential applications of solar lasers in space are emerging. These include earth, ocean, and atmospheric sensing from space; detecting, illuminating, and tracking hard targets in space; and deep-space communications. Gas, liquid, and solid lasers have all been considered as candidate solar lasers, and successful lasing has been achieved with the use of a number of such systems. It is predicted that solid-state lasing materials with broadband absorption characteristics such as alexandrite or Nd:Cr:GSGG might yield greater than 10% conversion efficiency.
High-performance satellites need massive on-board information-processing capability. Fiber-optic communication systems are expected to take their place naturally in the computers of spacecraft, as they do in ground-based systems. Optical information processing for on-board analysis and synthesis of sensor data is becoming increasingly important in satellites. NASA has programs in optical implementations of on-board signal processing, image processing for robot vision systems, and spatial light modulators as devices needed to achieve these objectives, as well as in fiber-optic communications. Long-range plans for lunar colonization and Mars exploration point up the need for these advanced optical technologies.
Optical sensors are needed in a variety of satellite applications. The fiber-optic rotation sensor, particularly in its integrated optics configuration, will be important in high-sensitivity, low-weight inertial navigation systems. Optical systems are being developed to assist in docking between satellites. Fiber optics and integrated optics sensors are also being considered for use in monitoring the health of a spacecraft.
Many spacecraft require accurate on-board clocks. For example, the cesium atomic clock is used on the Global Positioning System satellites. Recent developments in the field of atom-ion trapping and cooling suggest that with further research greatly improved spacecraft clocks can be developed that will offer much higher accuracies.
The space shuttle glows in the dark, and so do orbiting satellites. The glow is accompanied by erosion of the surfaces of the spacecraft and the instruments it carries. The glow complicates the design and operation of optical and infrared instruments, and the erosion limits their effective lifetime. The glow and erosion are produced by the impact of atmospheric ions and atoms with the spacecraft at velocities that correspond to energies of about 5 eV. This phenomenon results in a complex array of surface and gas-phase reactions that depend on the nature of the surface. Much further AMO research is, however, required in order to fully understand the phenomenon and minimize its effects.
One increasing problem in the space program is the growing amount of debris in orbit. Lasers have been suggested both as optical monitors of space
debris (through a form of laser radar) and as a means for elimination of small amounts of debris (either by vaporizing it or by deflecting it out of the satellite path). In either case, lasers must be placed in the satellites. Further research and development are required to determine the optimal design for such applications.
AMO science plays an important role in spacecraft operations and applications that will increase in the future with the advent of sophisticated atmospheric remote sensing systems and laser satellite communications.
The pace of our nation's commerce continues to rely heavily on an aging, yet critically important, transportation infrastructure. Unlike other segments of our national economy, the responsibility for this infrastructure rests heavily with federal, state, and local governments. At a time of increasing demands on scarce governmental resources, it is crucial that cost-effective technologies be available to improve the safety and effectiveness of transportation systems throughout the nation.
Although AMO research is not usually considered central to transportation issues, it nonetheless is playing an important role in air, land, and sea transportation through sensing and control of vehicle movement (navigation), through improvements in safety, and through increased fuel efficiency.
In aviation, lasers are being used to detect wind shear, clear air turbulence (CAT), and wake vortices. These phenomena can cause severe injury to passengers and damage to aircraft. For example, the 1987 crash of a Delta airlines jet at the Dallas–Fort Worth Airport was attributed to severe wind shear. High-sensitivity light scattering techniques developed by AMO researchers are being applied to the detection of wind shear. For instance, a LIDAR system can measure wind velocity through minute frequency shifts of laser light scattered from moving aerosol particles in the atmosphere. Wind shear velocities are in the range of 10 to 30 m sec-1 and give frequency shifts of 1 to 3 megahertz (MHz) for 10-µm carbon dioxide lasers and 5 to 15 MHz for 2-µm solid-state laser sources. LIDAR systems can determine wind shear and turbulence out to a range of 10 km in front of aircraft and give pilots sufficient warning to take appropriate avoidance action.
Measurement of air turbulence has implications for the efficiency of use of airport resources. Airport operators need information on air turbulence coming from wake vortices created during takeoff and landing of large transport aircraft.
Because of the danger from this turbulence, aircraft must be spaced sufficiently far apart that the wake of the preceding aircraft does not disturb the next flight; such spacing can produce a severe bottleneck at congested airports.
Likewise, collision avoidance through automated systems, or through a warning system activated by laser radars, would be a considerable boon in skies near airports. Lasers could also be used to detect damaging levels of particulates from the large, widespread ash clouds caused by erupting volcanos. In a different application, the use of laser bar code readers has been suggested as a means for automated optical identification of aircraft near to or on the ground. This could assist controllers in keeping track of air traffic.
There are numerous other ways in which lasers and electro-optics are becoming enabling technologies in aviation. Commercial and military aircraft have used laser gyros for a number of years. In addition, optical fiber communications and optical sensors will play an increasing role in the aircraft of the future and provide pilots with more information than they currently have, which will be fused into intelligent optical displays. The infrastructure of quantum electronics has provided building blocks for many of these advances.
In ground transportation, optical techniques for detecting speeders are coming into use. A laser-based system can unambiguously discriminate a particular car and give a rapid and precise reading of its speed. The system is compact, efficient, lightweight, and safe to use.
Several programs are under way to develop ''smart highways" and automated rail systems. Optical sensors identify the position of vehicles, and a systems analysis of a real-time optical display, suggesting alternate routes. Already used in a rudimentary form on highways, such systems would be dramatically improved by less expensive and more efficient display technologies.
Future generations of automobiles may use lasers and optical science in several places. Fiber optics has been used to demonstrate that the lighting systems required in an automobile all can be powered by a single light source. Inexpensive and localized laser radar systems may be used to assist in collision avoidance. Optical fiber sensors are being developed to monitor combustion within the cylinders of an automobile engine. Such sensors make it possible to optimize combustion within an operating automobile, improving efficiency. Although currently used only for research on improved automobile engines, someday such monitors may be implemented in all vehicles.
Laser-based CD systems are currently entering the market as sources of onboard maps for land vehicles and boats (and aircraft). When combined with inexpensive global positioning receivers, they can provide the operator of almost any vehicle reliable, detailed information on location and local terrain, road
networks, and so on (Figure 2.14). Such systems are being tested, for example, in rental cars in Florida.
AMO science is making a significant contribution to improvements in transportation systems and safety. The many new ideas currently under development suggest that this contribution will increase in the future