The adsorption of water on clean and oxygen-dosed Ru(011)

Abstract

Water adsorption on clean and oxygen-dosed Ru(001) has been examined using thermal desorption spectroscopy (TDS), AES, LEED, and electron stimulated desorption ion angular distributions (ESDIAD). On the clean Ru(001) surface, three water TDS peaks were observed at 215 K (A 1), 180 K (A 2) and 155 K (C). The structure of the water on the surface was determined by ESDIAD and LEED as a function of coverage and temperature. The (√3 × √3)R30° LEED pattern indicated that water is in registry with the Ru substrate which is responsible for the close match of the ice and Ru lattice spacings. ESDIAD patterns were able to determine the presence of water monomers at low coverages and temperatures and to distinguish between water structures on the surface as a function of temperature. Both the A 1 and A 2 TDS peaks were found to be due to the desorption of water molecules from two-dimensional clusters. The energy separation of the two peaks was related to the dipole-dipole interactions from water bound into the clusters in two different orientations. A complex temperature- and coverage-dependent LEED structure was also observed which was attributed to an ordered domain structure of the water clusters. Preadsorption of oxygen on Ru(001) inhibited the azimuthal ordering of the adsorbed water as determined by ESDIAD and LEED for all water coverages and oxygen coverages. Oxygen pre-exposures also had a strong influence on the TDS peaks. The strong interaction of water with adsorbed oxygen induced an unfavorable water-substrate geometry.