The adsorption of H₂O on clean and oxygen precovered Ni(111) studied by ARUPS and TPD

Using TPD, ARUPS, XPS, LEED and work function measurements the adsorption of water has been studied on the clean Ni(111) surface and on a Ni(111) surface precovered with 0.25 ML of oxygen. For the clean surface the saturation coverage of chemisorbed water is determined to 0.66 ML by XPS. The observed ( 3 × 3)R30° structure must then be non-primitive containing two molecules per unit cell. Angle resolved UPS spectra show a splitting of the 3a₁ level indicating the existence of two nonequivalent molecules in the chemisorbed layer, both being tilted with respect to the surface normal. The so-called "bilayer" structure is proposed for this saturated chemisorbed layer. The TPD spectra show three distinct peaks at 171, 152 and 158 K that are attributed to desorption from the chemisorbed bilayer, a transition layer and condensed water, respectively. Upon oxygen preadsorption an additional peak is observed in TPD of water reaching from 180 to 260 K with a saturation coverage of 0.25 ML. This peak is attributed to intact H₂O molecules directly chemisorbed on the Ni(111) surface. The angle resolved UPS spectra show very distinct angular dependences of all molecular levels. No evidence for dissociation is found. The orientation of the molecules is proposed to be tilted perpendicular to the molecular plane possibly due to hydrogen bonding to two neighbouring oxygen atoms.