Theory of initial yields of ions generated by electrons in binary mixtures. II

Eggarter [J. Chem. Phys. 84, 6123 (1986)] showed a method of calculation of the yield of ionization in a mixture by electrons, and carried out a numerical solution for the Ar--H2 mixture. At sufficiently high electron energies, the yields N/sup Ar/ and N/sup =/ of each component species closely follow the relation N/sup Ar//N/sup =/ = KxC/sup Ar//C/sup =/, where C/sup Ar/ and C/sup =/ represent the concentration fractions, and K is a constant. We now present a theoretical interpretation of that relation, and show the microscopic meaning of the constant K; it is the ratio of the ionization cross sections of the two component species, evaluated for an electron of a certain kinetic energy, which is much greater than the ionization threshold energy. The interpretation rests on the fact that the Spencer--Fano electron degradation spectrum depends on electron energy only mildly and smoothly except near the initial source energy and near the first ionization threshold energy. The same relation should hold approximately for any combination of ordinary molecules, although for the Ar--H2 case the relation is obeyed especially close. We also discuss the connection of our treatment with earlier treatments based on certain assumptions about the radiation-energy partition among the mixture components