Electron Excitation of Auger Transitions in Atoms

It has been observed experimentally that total ionization cross sections for neutral atoms exhibit abrupt increases near thresholds for exciting inner-shell electrons to autoionizing states. This feature of the ionization cross section is not predicted by most of the standard first-order theories. While a recent R-matrix calculation produced cross sections of the type observed experimentally, the physical effects contributing to the structure were not apparent. We have studied this effect in the distorted-wave approximation and have found that the rapid increase in the ionization cross section near autoionization thresholds results from exchange effects. Calculated angular distributions for Li, where this mechanism operates, are dominated at backward-scattering angles, a feature amenable to experimental observation