ANNIHILATION OF POSITRONS TRAPPED AT THE ALKALI-METAL-COVERED TRANSITION-METAL SURFACE

Recent studies of the Cu(100) surface covered with submonolayers of Cs [A. R. Koymen et al., Phys. Rev. Lett. 68, 2378 (1992)] revealed that the normalized intensity of the positron-annihilation-induced Cu M(2, 3) VV Auger signal remains nearly constant at the clean Cu(100) surfac e value until the Cs coverage reaches approximately 0.7 physical monol ayer, at which coverage the signal intensity drops precipitously. We p resent a microscopic analysis of this unusual behavior of the Cu M(2,3 ) VV Auger signal based on a treatment of a positron as a single charg ed particle trapped in a ''correlation well'' in the proximity of the surface atoms. The image-potential-induced positron surface states are calculated using the corrugated-mirror model in a full three-dimensio nal geometry. These states are studied for the clean Cu(100) surface a nd for various ordered structures of the Cs adsorbate on the Cu(100) s urface below and above the critical alkali-metal coverage of approxima tely 0.7 physical monolayer. Calculations show that whereas the positr on surface state is localized in the region of the Cs/Cu(100) interfac e for Cs coverages below the critical alkali-metal coverage, at a Cs c overage corresponding to one physical monolayer the positron surface s tate is localized on the vacuum side of the Cs overlayer. The probabil ities for a positron trapped in a surface state to annihilate with rel evant Cu and Cs core-level electrons as well as the positron surface-s tate lifetimes are computed for various alkali-metal structures on the Cu(100) surface and compared with experimental positron-annihilation- induced Auger-electron-spectroscopy data. It is shown that a shift in localization of the positron surface state from the Cs/Cu(100) interfa ce to the vacuum side of the alkali-metal overlayer results in a sharp decrease in the positron-annihilation probabilities with Cu 3s and 3p core-level electrons, in agreement with experiment.