HALOGEN CHEMISORPTION ON SILVER(111) - SCANNING-TUNNELING-MICROSCOPY OF COADSORBED HALOGEN ATOMS

The structures of halogen monolayers (I, Br, Cl, and F) and mixed-halo gen monolayers on Ag(111) surfaces are measured by scanning tunneling microscopy, (STM). Adatom radii determined from nearest-neighbor bond distances are in excellent agreement with literature values of the rad ii of halides, indicating that the adsorbed halogen atoms have an esse ntially ionic character and are adsorbed as a surface Ag-halide comple x. STM images of mixed-halogen adlayers, comprised of either coadsorbe d I and Cl, I and F, or Cl and F, exhibit several structural features, e.g., a low-frequency moire pattern and a large atom vacancy density, that are not observed in single-atom adlayers and that are ascribed t o Coulombic forces within the ionic adlayer. STM has also been used to monitor variations in adlayer structures following ion exchange of ch emically distinct halogen species. STM images and data obtained from A uger electron spectroscopy indicate that the chemical stability of the surface Ag-halide complexes in aqueous solutions decreases with incre asing halogen electronegativity. The results suggest that the energy o f solvation of the halide ions determines the relative stability of th e Ag-halide surface bond, similar to that observed for bulk silver hal ide crystals.