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ION PROCESSES, NEUTRAL CHEMISTRY, AND THERMOCHEMICAL DATA 51 Cross sections for these long-range Coulomb collisions are not particularly sensitive to the internal structure of the reactants, and therefore they are amenable to scaling laws based on the electron affinity and ionization potential of the reactants. For example, semiempirical cross sections for ion-ion neutralization at low pressure have been proposed by Moseley et al.12 Electron-Ion Recombination Collisions between electrons and positive ions resulting in neutralization of the ion constitute electron-ion recombination. The rates of these reactions scale with the square of the plasma density. Since electron-ion recombination results from a long-range Coulomb force, the cross section often scales inversely with energy. The total rate of electron-ion recombination reactions will therefore usually be ignorably small in the sheaths, and their cross sections (or rate coefficients) are required only in the thermal energy range. At the plasma densities and temperatures of interest for plasma processing, radiative and collisional radiative recombination are not important. Dissociative recombination of molecular ions is the only significant volumetric recombination process. The distribution of neutral products should be identified, as well as their translational energies. Cross sections for these processes are widely scattered in the literature, with few examples for systems of interest to plasma processing. Recent compendia and reviews of cross sections and rate coefficients can be found in Mitchell13 and Adams.14 The temperature dependencies of these processes are also important.15 Ion-Neutral and Neutral-Neutral Excitation Inelastic collisions between ions and neutral species that result in excitation, dissociation, or ionization of the neutral target without transfer of charge are classified as ion-neutral excitation collisions. Hot atom collisions on neutrals can similarly instigate excitation, ionization, or dissociation. Since there is a threshold energy associated with these collisions, they are important only for superthermal ions and usually only in the sheath regions. Similar reactions involving hot atoms may occur throughout the bulk plasma. There is fragmentary data scattered through the literature for energetic ion-neutral and neutral-neutral excitation collisions. Phelps16 has compiled and assessed complete cross section sets (see Figure 6.3) for ion and high-energy neutral impact reaction mechanisms for Neutral Chemistry Many neutral chemistry databases have been developed for combustion and atmospheric chemistry?17 Processes for evaluation of those databases are well established, and so evaluation is not addressed here. Instead, the panel assesses the status of the availability of rate coefficients and proposes methods to address unmet needs. It is important to emphasize that these needs are better stated in terms of mechanisms, as opposed to a collection of rate coefficients.
