Abstract
The impact-parameter dependence of direct K vacancy production in adiabatic ion-atom collisions calculated with the semiclassical approximation and in the Briggs model (1975) are compared. In the asymmetric limit the two approaches agree. For more symmetric collisions the Briggs model gives results in agreement with recent experiments for Xe on Pb. In particular it reproduces a dip in the ionisation probability at small impact parameters.