ADSORPTION SITE CHANGE FOR CS, RB OR K ADSORPTION ON AG(111)

The adsorption geometries for the primitive (3 x 3), (2 x 2) and (root 3 x root 3)R30 degrees structures of K, Rb and Cs on Ag(111) have bee n determined using low-energy electron diffraction. In the lower-cover age (3 x 3) and (2 x 2) structures, the adatoms occupy fcc hollow site s, while in the (root 3 x root 3)R30 degrees structure they occupy the hcp hollow sites. The fcc hollow structures are accompanied by signif icant substrate rumpling. There is no significant coverage-dependent o r site-dependent change in chemisorption bond length. However, there i s a large coverage-dependent anisotropy of vibrational amplitude of th e adatoms, with the parallel component as much as five times larger th an the perpendicular component at low coverages.