STRUCTURAL INVESTIGATION OF ORDERED SB ADSORPTION PHASES ON AG(111) USING COAXIAL IMPACT COLLISION ION-SCATTERING SPECTROSCOPY

The ordered overlayer structures formed following the adsorption of Sb on Ag(111) have been studied using coaxial impact collision ion scatt ering spectroscopy (CAICISS). At coverages up to several atomic layers there is evidence for layer-by-layer growth at room temperature, alth ough the individual layers show no long-range order. Subsequent anneal ing gives rise to two ordered phases, a (root 3 x root 3)R30 degrees-S b and a (2 root 3 x 2 root 3)R30 degrees-Sb. Neon ion CAICISS data hav e been used to distinguish between overlayer and substitutional struct ural models for the (root 3 x root 3)R30 degrees-Sb phase, and pure su bstitutional and mixed substitutional/overlayer models for the (2 root 3 x 2 root 3)R30 degrees-Sb phase. Despite the complexity of multiple scattering contributions, these data clearly favour the substitutiona l adsorption site for the (root 3 x root 3)R30 degrees-Sb phase giving a surface alloy of the form Ag2Sb. For the (2 root 3 x 2 root 3)R30 d egrees-Sb phase, the data are best described by a model involving an o rdered p(2 x 2)-Sb overlayer (periodicity relative to the substrate) s uperimposed on the (root 3 x root 3)R30 degrees-Sb surface alloy.