Image acceleration of highly charged ions on metal, semiconductor, and insulator surfaces

Very slow, highly charged ions impinging on metal surfaces are known to be accelerated by their image and to drop irremediably on the surfaces which they touch or slightly penetrate. We present experiments which demonstrate that above insulators or semiconductors, at normal incidence, the ions are backscattered at a certain distance from the surface and do not touch it. This finding is explained by the transient buildup of positive charges due to the removal of valence electrons, which overcome the acceleration of the ion by its own image. This effect is found to cancel out at grazing incidence.