Impact-parameter-dependent electronic stopping of swift ions. 1. Binary theory

A computational scheme has been developed to estimate the mean electronic energy loss of an incident swift ion on an atomic target as a function of the impact parameter between the moving nuclei. The theoretical basis is binary stopping theory. In order to extract impact-parameter dependencies it was necessary to incorporate the spatial distribution of the target electrons. This distribution is immaterial for the stopping cross section and straggling parameter. Incorporating it into the existing formalism involves additional numerical integrations. The emphasis in the present paper is laid on verifying the reliability of the scheme. Existing theoretical estimates with comparable input are based on the Born approximation and, more or less explicitly, refer to incident protons. Since the present estimates are based on classical stopping theory, a rough inverse-Bloch correction has been developed to ensure a meaningful comparison. Good agreement is obtained in general, and where discrepancies are found, their origin, whether in the present scheme or the Born approximation, is discussed. The formalism incorporates the Barkas-Andersen effect as well as screening and shell corrections. While these effects play determining roles in the stopping cross section, illustrating their role in the impact-parameter dependence reveals interesting qualitative features, in particular in the dependence on ion charge. (authors)