| Themes > Science > Physics > Molecular Physics > Excitation Energy Transfer and Energy Migration > Momentum Transfer, Swarm, and Disharge Measurements |
For purposes of modeling electrical discharges in gases, it may not be possible or even necessary to use complete, detailed information about the dynamics of electron-molecule collision processes that produce dissociation. It may be sufficient to consider only total or averaged "effective" cross sections that apply to 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 data 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. 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 H2O to form HF. |
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