INSITU SURFACE EXAFS STUDY OF AN UNDERPOTENTIALLY DEPOSITED SILVER MONOLAYER ON GOLD (111)

Fluorescence detected surface extended x-ray absorption fine structure (EXAFS) has been used to probe the structure of underpotentially deposited Ag on a Au(111) surface from an aqueous solution using sodium perchlorate as the electrolyte. Silver is shown to be fully reduced on the surface with observed bond distances of 2.88 +/- 0.025 angstrom and 2.61 +/- 0.025 angstrom for silver-silver and silver-gold near neighbors, respectively. Data is obtained with the polarization of the incident x-ray beam both parallel and perpendicular to the electrode surface. From this we show that the silver monolayer is (1 x 1) commensurate with the substrate and that the Ag ad-atoms occupy three-fold hollow sites. In addition to backscattering from gold and silver near neighbors, backscattering from oxygen, presumably adsorbed water is observed. We have studied this at different electrode potentials (+0.7, +0.5, -0.1 V vs. Ag/AgCl). At all three potentials the measured silver-oxygen distance is the same: 2.21 +/- 0.025 angstrom. This is contrasted to another system where this previously has been studied, Pb on Ag(111), where the metal-oxygen distance varies with applied potential.