THEORETICAL-STUDY OF THE POSITRON SURFACE-STATE AT AN ALKALI-METAL SURFACE

We present a quantum mechanical analysis of positrons trapped in a bou nd state at the (100) surface of bulk Cs, based on the treatment of a positron as a single charged particle trapped in an image potential-co rrelation well in the proximity of the surface atoms. The image-potent ial-induced positron surface state is calculated using the corrugated- mirror model in full three-dimensional geometry. Calculations show tha t the positron bound state is delocalized in the surface plane, but is strongly localized perpendicular to the surface mainly in the region between the top and the second layers of Cs atoms. The positron bindin g energy E(b) in the surface state is computed to be 4.92 eV. Calculat ions on the positron annihilation rates for different Cs core levels a re performed using the independent particle model. The positron surfac e state lifetime is calculated using the local density approximation m ethod. The annihilation probabilities of a positron trapped at the Cs( 100) surface with Cs 4p and 4d core electron levels are computed to be 0.065% and 0.22%, respectively.