Observation of Hollow Atoms or Ions above Insulator and Metal Surfaces.

We present some experimental results which demonstrate that hollow atoms (ions) can be formed above insulator surfaces, and show for the first time dramatic differences in the interactions of a given ion with a metal and a semiconductor (insulator) surface, leading to the formation of different hollow atoms (ions). These results are tentatively explained in considering the localized (valence) or delocalized (conduction) character of the captured electrons and the backscattering of the ions above insulators. {copyright} {ital 1996 The American Physical Society.}