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 been determined using low-energy electron diffraction. In the lower-coverage (3 x 3) and (2 x 2) structures, the adatoms occupy fcc hollow sites, while in the (root 3 x root 3)R30 degrees structure they occupy the hcp hollow sites. The fcc hollow structures are accompanied by significant substrate rumpling. There is no significant coverage-dependent or site-dependent change in chemisorption bond length. However, there is a large coverage-dependent anisotropy of vibrational amplitude of the adatoms, with the parallel component as much as five times larger than the perpendicular component at low coverages.