Emission of band-gap-energy positrons from surfaces of LiF, NaF, and other ionic crystals.

Energetic positrons implanted into various ionic crystal surfaces are reemitted with a maximum kinetic energy (14.7+-0.7eV for LiF) near the band-gap energy. The evidence suggests that, at least for LiF and NaF, the emission originates from bulk- formed positronium (Ps) breaking up after diffusing to the surface. In our model, the positrons are Auger-emitted when the Ps electrons recombine with holes at the surface, and the spectrum of positron energies thus reflects the density of empty surface states.