Study of the higher-order corrections to the impact-parameter dependence of energy loss

Abstract

A method is proposed to describe the impact-parameter dependence of energy loss in ion–atom collisions, ΔE(b), which is based on the local plasma frequency (LPF) approach. In this method, a linear response approach is combined with an explicit description of the energy loss to a free electron. This results in a general scheme of calculation where both the Barkas and Bloch corrections are presented. The calculated stopping cross-sections are in satisfactory agreement with the relevant experimental results. However, a serious disagreement was found by comparing the calculations with our recent results for energy losses in a thin gold foil, measured as a function of exit angle. This data reflect the impact-parameter dependence of energy loss in single ion–atom collisions. To explain this disagreement we address the problem of proper conversion of the impact parameter scale to the angle of deflection in single ion–atom collisions. It is argued that, due to specific quantum effects, the deflection angle for a given impact parameter depends significantly on the eventual energy loss. Under the conditions used in the experiment this can lead to an increase by a factor of two of the variation of the energy loss in a foil as a function of exit angle.