Sources of Data and Interpretations

This section consists mostly of a discussion of how we are to interpret and reconcile the electron collision cross sections that we obtain from our three primary, but disparate, sources of such data and how these interpretations are related to the modeling and simulation for which we are assembling the basic collision data in the first place. 

All our electron impact cross-section data arise trom three sources  

Ab initio quantum theoretical calculations (Huo and Gianturco, 1995 Winstead and McKoy, 1999), in which Schrodinger s wave equation is solved in some approximation appropriate to the scattering problem and 

Electron swarm measurements (Huxley and Crompton, 1974), in which a burst of electrons is observed to drift along an electric field applied to a low-density gas and various transport coefficients, such as the drift velocity, transverse or longitudinal diffusion coefficients, attachment or ionization coeffieients, and so on, are measured as functions of the applied electric field divided by the pressure or the gas number density (i.e., E/p or E/N) collision cross sections, which are related to the transport coefficients through Boltzmann s transport equation (Morgan, 1979 Morgan and Penetrante, 1990), can be extracted by a process of inversion. 

Beam collision measurements represent the ideal for us in terms of potential quality of data, but they are the most scarce in terms of quantity. Many early beam measurements were of relative cross sections nevertheless, they are useful when used in conjunction with calculations or swarm measurements. Ab initio calculations of electron impact cross sections for complex molecules, as discussed by Winstead and McKoy (1999), have become very sophisticated but require enormous computational resomces for large molecules. The third technique has been in use for some three decades. There is a very large body of literature reporting on measurements and interpretations of electron transport or swarm coefficients in many of the same gases in which we are currently interested. This is an excellent technique, as I describe below, for estimating cross sections when no other data are available. 

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