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ELECTRON COLLISION PROCESSES 44 a particular discharge condition specified, for example, by electric field to gas density ratio (E/N) or local temperature. Relatively reliable total electron collision cross sections can often be derived from analysis of dam from drift-tube or simple discharge experiments. The approach used requires that there be a reasonably well understood self-consistent model for the transport of electrons that can account for observations of such parameters as the electron drift velocity, ionization growth, the longitudinal and transverse diffusion coefficients, the relative intensity of observable atomic or molecular emissions, and so on. The model must be capable of predicting the kinetic energy distribution of the electrons, e.g., from numerical solution of the Boltzmann transport equation or from a Monte Carlo simulation, and must also be consistent with independently determined cross sections (such as ionization cross sections) that are known to be reliable. This method is most often used to derive momentum transfer cross sections, and inelastic cross sections for low threshold processes such as rotation and vibration. There are numerous compendia of momentum transfer cross sections derived in this fashion.10 This approach has also been used successfully to generate fairly complete collision cross section data sets for several atomic and molecular species. However, except for a few cases such as SF6, it has generally been difficult to distinguish dissociation processes from other types of molecular excitation processes. With the advent of more sophisticated diagnostics such as two-photon laser-induced fluorescence, it is possible to measure the densities of neutral dissociation fragments, from which dissociation rates can possibly be extracted. Information about reactive fragment densities can also sometimes be determined indirectly from examining the products of subsequent neutral chemistry in which these species are involved, e.g., fast reactions of H with NO2 to form NO and of F with H20 to form HF. GENERAL COMMENTS Of the major system constituents selected as examples, consider SiH4. Silane has been the object of intense study, especially during the last 5 years.11 Fundamental studies have provided cross sections for dissociative ionization, neutral attachment, and dissociative attachment. Cross sections are not available for excited-state targets. Carbon tetrafluoride can be described as a success story, in that it has been the subject of intense study and pertinent data are well known.12 Accurate cross sections have been measured for dissociative ionization (the parent ion CF4+ is unstable). A report of recent measurements on neutral dissociation has been published recently.13 Cross section measurements have been reported for parent ionization of the radicals CFx (x = 1-3) and dissociative ionization of CF3. Measurements have also been made for dissociative attachment (negative ion formation). The picture is less complete for the other species in the example set. For the atoms of interest, ionization and excitation cross sections are known. For C12 and F2, only total ionization and excitation cross sections are known. Vibrational excitation and momentum transfer cross sections have been calculated. No data were found on dissociative ionization or neutral dissociation. For Br2, no data have been reported for. For HCl, cross section sets have been derived from swarm data. Dissociative attachment cross sections have been measured. For HF, vibrational excitation cross sections have been measured and dissociative attachment cross sections derived. No additional data are available, and no complete data set has been proposed. No data were found for HBr and only limited data exist for BCl3.14 By applying existing techniques, one could in principle obtain full data sets for many compounds. The existing database is most complete for those processes that are least relevant for the modeling and diagnostics of processing plasmas, whereas there is a serious lack of reliable experimental data for the most important processes (dissociation into neutral ground-state fragments, data for free radicals, data for vibrationally excited molecules and radicals, and data for excited target species).
