Ionization probability of sputtered indium atoms: Dependence on projectile impact angle

Motivated by recent model calculations describing secondary ion formation during sputtering of a metallic surface, we have investigated the dependence of the ionization probability of atoms sputtered from clean indium surfaces on the projectile impact angle. For that purpose, the secondary ions and their neutral counterparts were detected under otherwise identical experimental conditions using time-of-flight mass spectroscopy in combination with VUV laser post-ionization. It is found that the average ionization probability depends on the impact angle of the projectile ion beam in such a way that more oblique incidence leads to a larger ion fraction. These preliminary results, which corroborate a corresponding prediction from the model calculation, are interpreted in terms of the kinetic substrate excitation induced by the impinging projectile.