Screening effects in the impact parameter dependence of electronic energy loss offast ions in collisions with atoms

Abstract

The impact parameter dependence of electronic energy loss of fast ions in ion-atom collisions is studied theoretically. Effects of screening of the nuclear charge of the ion by its own electrons are discussed. Besides the pure electrostatic screening the effect of the possible excitation or ionization of electronic shells of the projectile during the collision (dynamic screening) is considered. The calculations have been made for He q+ collisions with Al and Au atoms within the framework of the semiclassical approximation. It has been assumed that the energy loss is mainly due to ionization of the target atoms. The bound-bound transitions in the target have been ignored. It is demonstrated that both types of screening (static and dynamic) considerably change the impact parameter dependence of the mean energy loss as compared with that for the bare nucleus. The effective charge of the projectile as a function of the impact parameter is calculated and discussed.