The adsorption geometry of Ag(111)-(root 7 x root 7)R19.1 degrees-4Ar studied by LEED

The adsorption geometry of Ag(1 1 1)-(root7 x root7)R19.1 degrees -4Ar was studied using low-energy electron diffraction (LEED). This higher-order com mensurate 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 overla yer. The dynamical LEED data were taken at a sample temperature of 31 K, an d the results of the REED analysis indicate that one Ar atom per unit cell occupies the top of a Ag substrate atom, while the other three Ar atoms occ upy bridge sites. There is little, if any, distortion of the Ar lattice fro m hexagonal symmetry. The average Ar-Bg perpendicular spacing in this struc ture is 3.22 +/- 0.07 Angstrom. The Ar-Ag nearest-neighbor distances are 3. 22 and 3.52 Angstrom for top-site and bridge-site Ar atoms, respectively. ( C) 2001 Elsevier Science B.V. All rights reserved.