| Abstract
| - The adsorption of H2O on the oxygen-terminated polar surface of ZnO, ZnO(000-1), has been studied by Heatom scattering (HAS), low-energy electron diffraction (LEED), adsorption probability measurements, Heatom reflectivity measurements as a function of exposure and surface temperature (He atom thermal desorptionmeasurements, “He-TDS”), and X-ray photoelectron spectroscopy (XPS). The clean O−ZnO(000-1) surfaceis characterized by an ordered (1 × 3) oxygen vacancy structure which converts to a (1 × 1) hydrogen(OH)-terminated structure upon dissociative H2O adsorption, even at adsorption temperatures as low as TS =200 K. The formation of the OH-species is accompanied by the formation of a shoulder in the XPS O 1s line.A detailed investigation of the coverage dependence of the H2O adsorption probability indicates the presenceof a distinct precursor state. The initial trapping probability is S0 = 0.8 ± 0.1. The most probable microscopicadsorption mechanism which is consistent with the obtained data is a trapping of the molecules in a precursorstate and a subsequent dissociation at O vacancy sites, yielding two OH-species per dissociated H2O moleculeon the surface. The binding energy of the OH-species amounts to ∼130 kJ/mol as determined from He-TDScurves.
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