Distorted-wave theory of electron-ion collisions. II. Auger ionization and excitation fluorescence

Abstract

When electrons of moderately high energy collide with multiply stripped ionic targets, the predominant mode of collision is the impact excitation of outer-shell electrons, which subsequently decay by radiative emission. As the degree of ionization increases, the strength for inner-shell excitation becomes comparable with that for the direct-impact ionization of outer-shell electrons. The inner-shell vacancy thus created may decay by emission of an electron [Auger ionization (AI)] or a photon [excitation fluorescence (EF)]. For highly charged ionic targets, it is shown that the Auger ionization contributes significantly to the total ionization cross section, and becomes dominant over the direct-impact ionization in some cases (${\mathrm{Mo}}^{24+}$). The excitation probabilities, both to discrete and continuum states, have been calculated for the inner- and outer-shell electrons, using the improved Bethe approximation and the semiclassical projection operator. The result shows the drastic decrease in the relative transition strength to the continuum as the degree of ionization $Z_I$ increases. The branching ratios for the Auger ionization and fluorescence decay are fitted as functions of $Z_I$ for ionized targets. The AI and EF cross sections are compared with the corresponding direct processes.