Adsorption geometry of Ag(111)-(root 7 x root 7)R19.1 deg -4Ar studied by LEED

The adsorption geometry of Ag(111)-(root 7 x root 7)R19.1 deg -4Ar was studied using low-energy electron diffraction (LEED). This higher-order commensurate phase was produced by preadsorbing a small amount of CO in order to block the close-packed steps, which pin the rotation angle of the overlayer. The dynamical LEED data were taken at a sample temperature of 31 K, and the results of the LEED analysis indicate that one Ar atom per unit cell occupies the top of a Ag substrate atom, while the other three Ar atoms occupy bridge sites. There is little, if any distortion of the Ar lattice from hexagonal symmetry. The average Ar-Ag perpendicular spacing in this structure is 3.22 plus/minus 0.07 A. The Ar-Ag nearest-neighbor distances are 3.22 and 3.52 A for top-site and bridge- site Ar atoms, respectively.