SCREENING EFFECTS IN THE IMPACT PARAMETER DEPENDENCE OF ELECTRONIC-ENERGY LOSS OF FAST IONS IN COLLISIONS WITH ATOMS

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 ex citation or ionization of electronic shells of the projectile during t he collision (dynamic screening) is considered. The calculations have been made for He(q+) collisions with Al and Au atoms within the framew ork of the semiclassical approximation. It has been assumed that the e nergy 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 wi th that for the bare nucleus. The effective charge of the projectile a s a function of the impact parameter is calculated and discussed.