Growth of H2O layers on an ultra-thin Al2O3 film: from monomeric species to ice

Adsorption of water on an ultra-thin alumina film grown epitaxially onto NiAl(1 1 0) has been investigated by temperature-programmed desorption, work function measurements, ultraviolet photoelectron spectroscopy, and high-resolution X-ray photoelectron spectroscopy as a function of surface coverage at 100 K. At low coverages monomeric H2O species are adsorbed and it is proposed that the oxygen lone pair orbitals interact weakly via polarisation forces with the Al3+ cations of the second layer. No indication for the dissociation of H2O and the formation of OH− species has been found. For intermediate coverages clustering of H2O molecules occurs and the growth of a three-dimensional ice layer is observed. Care has to be exercised in photoemission experiments of ice layers, since UV photon-induced dissociation of H2O in thicker ice layers is indicated.