POSITRON-ANNIHILATION-INDUCED AUGER-ELECTRON-SPECTROSCOPY STUDIES OF PROPERTIES OF AN ALKALI-METAL OVERLAYER ON THE CU(100) SURFACE

Positron-annihilation-induced Auger-electron spectroscopy (PAES) emplo ys positrons trapped at the surface to create core-holes and to initia te the Auger process in atoms in the topmost layer of the surface. The results of experimental and theoretical investigations of the attenua tion of the positron-annihilation-induced Cu M2,3 VV Auger signal with Cs coverage on the Cu(100) surface at low and high temperatures are d iscussed. They reveal that at 163 K the normalized intensity of the po sitron-annihilation-induced Cu M2,3 VV Auger signal remains nearly con stant at the clean-surface value until the Cs coverage reaches approxi mately 0.7 physical monolayer, at which point the signal intensity dro ps precipitously. We present an analysis of this unusual behavior usin g a model that treats the positron as trapped in a double-well potenti al in the direction perpendicular to the surface: one well is associat ed with the Cu substrate and the other with the Cs adsorbate. The shar p drop in the Cu PAES intensity which occurs over a small change in th e 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. Th is migration can be accounted for in terms of a structural phase trans ition in the Cs overlayer from a disordered distribution of adsorbate atoms to adsorbate metallic islands with an ordered hexagonal close-pa cked structure.