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

A method is proposed to describe the impact-parameter dependence of energy loss in ion-atom collisions, Delta E(b), which is based on the local plasma frequency (LPF) approach, In this method, a linear response approach is co mbined 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 B loch 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 rec ent results for energy losses in a thin gold foil, measured as a function o f exit angle. This data reflect the impact-parameter dependence of energy l oss 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 spe cific quantum effects, the deflection angle for a given impact parameter de pends 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 va riation of the energy loss in a foil as a function of exit angle. (C) 2000 Elsevier Science B.V. All rights reserved.