The adsorption of water and oxygen on Ag(110): A study of the interactions among water molecules, hydroxyl groups, and oxygen atoms

Abstract

The adsorption of water on clean and oxygen covered Ag(110) in ultrahigh vacuum was studied with thermal desorption spectroscopy (TDS), electron stimulated desorption ion angular distribution (ESDIAD), low energy electron diffraction (LEED), work function measurements (Δ), and Auger electron spectroscopy (AES). The ranges of experimental conditions were θ(O) between 0 and 0.5, θ(H2O) between 0 and 2, and temperature between 80 and 400 K. Water adsorbs on the clean surface at 80 K with no long-range order and little, if any, short-range order as determined by LEED and ESDIAD, respectively. Electron bombardment of water adlayers on clean Ag(110) results in dissociation to form OH groups at 80 K. Water also reacts with preadsorbed oxygen, either in atomic or molecular form, to form OH groups at 80 K. The O-H bond axis of the OH groups is tilted with respect to the surface normal and its thermodynamically stable, azimuthal orientation is along the [001] direction. In the absenoe of excess, molecular H2O, the OH groups exhibit no hydrogen bonding. The OH groups form (1×m) (m=2,3) LEED compression patterns upon annealing to 200 K. The γ adsorption state, which evolves water in a desorption peak at 225 K, was carefully studied. This state consists of coadsorbed H2O and OH in a 1:2 ratio. The results of this work are combined with earlier TDS and vibrational measurements for this system to propose an adsorption model for the γ state, in which an H2O molecule is hydrogen bonded to two OH groups to form an (HO)2-H2O surface complex.