Adsorption of H 2O on clean and oxygen-preposed Ni(110)

Abstract

The adsorption of H 2O on both clean and modified Ni(110) surfaces has been studied using a variety of methods: electron stimulated desorption ion angular distribution (ESDIAD), thermal desorption spectroscopy (TDS), and low energy electron diffraction (LEED). Fractional monolayers, θ( H 2 O) < 0.5, of H 2O on clean Ni(110) are associated with a four-spot ESDIAD pattern suggesting that the H-ligands are in specific registry with the substrate. We postulate the formation of H 2O dimers bound via oxygen lone pair orbitals to Ni substrate atoms and oriented with the O … HO axis in [001] azimuthal directions. For θ( H 2 O) > 0.5–1 larger hydrogen bonded clusters with long range c(2 × 2) symmetry are formed. Upon heating to ⩾ 200 K, a fraction of the H 2O dissociates, forming OH(ad). TDS of H 2O from clean Ni(110) reveals four binding states having peak temperatures of 155, 210, 245 and 370 K. They are related to multilayer desorption (155 K), desorption from larger bilayer clusters (210 K), desorption from H 2O dimer clusters which might be stabilized by OH (245 K), and recombination of OH to yield H 2O(g) (360 K). Dissociation of H 2O is promoted by surface oxygen. For the adsorption of H 2O on oxygen-dosed Ni(110) at θ( O) > 0.08, a mixture of molecular and dissociative adsortion occurs immediately at 80 K, producing inclined OH. Isotropic exchange of H 2 16O with 18O(ad) is observed even for binding states in which dissociation is believed not to to occur and is related to a proton exchange involving H 2O(ad) hydrogen bonded to O(ad).