A SCANNED-ANGLE AND SCANNED-ENERGY PHOTOELECTRON DIFFRACTION STUDY OF (ROOT-3X-ROOT-3)R30-DEGREES AG ON SI(111)

Photoelectron diffraction data for Ag 3d emission from (square-root 3- x square-root 3)R30-degrees Ag on Si(111) have been obtained in all three possible modes: scanned polar angle, scanned azimuthal angle, and scanned energy. Single and multiple scattering cluster calculations with spherical-wave scattering have been carried out for a wide range of structural models and compared to experiment using R factors. In addition, scanned-angle photoelectron diffraction data for the substrate Si2p intensities have been obtained over essentially the full solid angle above both the clean and Ag-covered surfaces. These data and their analysis indicate that the Ag cannot be more than 0.5 angstrom below the topmost layer of atoms in the structure, and that the most likely models for the geometry among the many proposed are the honeycomb-chained-trimer and the two-domain missing-top-layer Ag honeycomb. We also suggest that these two models may coexist and interconvert between one another with subtle changes in the local Ag coverage between 2/3 and 1 ML.