Solvation and ionization of hydroxyl groups in water-ice layers on silver (110)

The solvation and ionization of hydroxyl groups on Ag(110) were examined wi th isotope exchange experiments involving OD and (OD)-O-18 and temperature programmed desorption (TPD). Water adsorption onto oxygen-covered Ag(110) g ives rise to the well known alpha, beta, gamma and delta peaks in TPD. The alpha-state represents multilayer water, the gamma state an (OH)(2) . H2O c omplex, and the delta-state OH groups. Surface solvation of OH groups invol ves as many as 14+/-3 water molecules (beta and gamma) in the limit of zero OH coverage and decreases linearly to five water molecules for 0.15 monola yer (ML) of OH. The solvating molecules are essentially all beta-state mole cules as the gamma-state contains only 0.5 water molecules per OH group. Th e maximum coverage of solvating water molecules and OH groups is 1.04 ML in good agreement with a perfect bilayer coverage of 1.18 ML on Ag(110). From this we identify the beta-state as an extended surface bilayer structure. Isotope exchange experiments demonstrated proton mobility in the OD/H2O adl ayer at temperatures of 150 K as well as migration of (OD)-O-18 groups from the surface and into alpha-state, multilayer water molecules. This constit utes desorption of hydro:ride ion into the water-ice multilayers and subseq uent solvation. (C) 1999 Elsevier Science B.V. All rights reserved.