Post-collision interaction in the excitation of helium by electrons

Abstract

The electron impact excitation of a large number of helium states has been studied in the energy domain where the autoionizing states lie. Apart from the well known negative-ion resonances, several structures are observed which can be explained in terms of a post-collision interaction (PCI) between the inelastically scattered electron and the electron ejected by an autoionizing state. It is found and made plausible that the PCI mechanism preferentially leads to the excitation of S states. From the experimental results it is possible to extract the dependence of the energy exchange between the scattered and the ejected electron on the excess energy of the scattered electron immediately after the excitation of the autoionizing state. From the magnitudes of the PCI structures in the excitation of states with different orbital momenta, conclusions can be drawn about the orbital momentum exchange in the PCI process. A brief discussion follows of the different theoretical approaches that have previously been proposed.