Abstract
The study of the interaction of positrons with surfaces is important because of the effectiveness of using positron beams as analytical tool to study surfaces. Low energy positrons that have been implanted into a solid target in a vacuum form a unique and sensitive probe of the surface regions of the solids [1-71. Of particular interest is the abundant positronium formation in the outer surface region. In the experiments a monoenergetic beam strikes a well-characterized single- crystal surface. The majority of the positrons penetrates the solid and simultaneously loses their energy rapidly within a characteristic stopping distance. The thermalized particles are fairly mobile and a large fraction of them eventually diffuse back to the entrance surface. Some materials have a negative work function for positrons, i.e., positrons are spontaneously re-emitted from the surface. A large fraction of the positrons reaching the surface may capture an electron and thus emerge as positronium rather than a positron.