Unexpected adsorption sites for potassium and rubidium adsorption on Ag(111)

Low-energy electron diffraction (LEED) results are presented for Ag(111)-p(2x2)-k, Ag(111)-(root 3x root 3)R30 degrees-K, Ag(111)-p(2x2)-Rb, and Ag(111)-(root 3x root 3)R30 degrees-Rb. In the p(2x2) structures, the adatoms occupy the fcc sites, while in the (root 3x root 3)R30 degrees structure they occupy the hcp sites. The p(2x2) structures include significant substrate rumpling, the effect of which seems to be to allow the adatoms to push more deeply into the substrate. The change of site from fee to hcp also seems to be related to the coordination of the chemisorption bond. The chemisorption bond length does not change significantly between the coverages studied here. Significant anisotropy of the vibrational amplitude was noted, with the in-plane vibrations being as much as three times larger than the out-of-plane vibrations.