Interaction of slow highly charged ions with surfaces

A review will be presented on recent investigations concerning the interaction of slow (≤ 106 m/s) ions in high charge states approaching a clean metal surface. Detailed information on the generation and decay of transiently formed multiply excited open-quotes hollow atomsclose quotes can be gained from the measurement of total yields and energy distributions of emitted electrons and, in particular, from the electron emission statistics. By comparing measured results with model calculations based on a recently extended classical over-barrier approach, different sources for the observed electron emission can be identified: autoionisation of the multiply excited hollow atoms on their way toward the surface; promotion above the vacuum barrier of electrons previously captured by the projectile, due to their self- and image-charge screening near the surface; 'peeling-off' of electrons still bound in highly excited projectile states at the moment of surface impact, and finally; electron emission due to final subsurface de-excitation