Positron-annihilation-induced Auger-electron spectroscopy (PAES) employs positrons trapped at the surface to create core-holes and to initiate the Auger process in atoms in the topmost layer of the surface. The results of experimental and theoretical investigations of the attenuation of the positron-annihilation-induced Cu ${\mathit{M}}_{2,3}$VV Auger signal with Cs coverage on the Cu(100) surface at low and high temperatures are discussed. They reveal that at 163 K the normalized intensity of the positron-annihilation-induced Cu ${\mathit{M}}_{2,3}$VV Auger signal remains nearly constant at the clean-surface value until the Cs coverage reaches approximately 0.7 physical monolayer, at which point the signal intensity drops precipitously. We present an analysis of this unusual behavior using a model that treats the positron as trapped in a double-well potential in the direction perpendicular to the surface: one well is associated with the Cu substrate and the other with the Cs adsorbate. The sharp drop in the Cu PAES intensity which occurs over a small change in the Cs coverage is attributed to a migration of positrons trapped at low Cs coverages at the Cs/Cu interface to the positron surface state on the vacuum side of the alkali-metal overlayer at high Cs coverages. This migration can be accounted for in terms of a structural phase transition in the Cs overlayer from a disordered distribution of adsorbate atoms to adsorbate metallic islands with an ordered hexagonal close-packed structure.

• Received 20 September 1993

DOI:https://doi.org/10.1103/PhysRevB.49.10577