Recent developments in electron degradation theory; selected topics

A main task of electron degradation theory consists of computing the degradation spectrum. This is essentially an energy distribution function of an irradiation system providing a general basis for determining not only the fate of incident (or internally produced) radiation but for determining the initial or primary yield of the various excited states of medium. For the simple case of a particle of kinetic energy T traversing a thin layer of material of thickness Δ x and molecular number density n, the formula for calculating the number of excitation of type ''s'' is given, using the cross section for the excitation of state ''s''. This is the definition of the degradation spectrum. Calculation of the yield of ions requires as input the secondary electron ejection cross sections for low-energy collision processes. The total ionization cross section is one of the most fundamental quantities in molecular physics. The gross apportionment of the energy loss into either ionization or excitation is more important than the distribution of energy of the secondary electrons ejected by primaries with impact energy less than a few hundred eV. A substantial body of cross-section data now exists for elastic collision and vibrational excitation for simple species in gas phase. For liquids, the subexcitation distribution has yet to be determined theoretically, and the calculation of even the rate of energy loss per unit path length for all modes remains as a difficult problem. (Yamashita, S.)