Geometry of the Ag{001}-c(2×2)Clstructure as determined by He diffraction

The structure of the Ag{001}-$c(2\mathrm{}\times 2)\mathrm{Cl}$ surface has been studied with He-atom diffraction. Two proposed Cl binding geometries have been considered as structural alternatives: the fourfold-hollow simple overlayer model and a mixed layer of coplanar Ag and Cl. We have calculated the repulsive He potential, based on self-consistent charge densities of the target, for both configurations. The two geometries are easily distinguished. The charge densities for the overlayer structure yield a corrugation which is in good agreement with the scattering data and lead to an unambiguous selection between the two structural models. We have further examined the possibility that the Cl overlayer undergoes a structural phase transition or irreversibly disorders at elevated temperatures. We find no change in the structure for temperatures up to 650 K, at which point Cl leaves the surface.