We have studied the electronic structure of ${\mathrm{H}}_2$O adsorbed on different metal surfaces between 7 and 200 K using photoelectron spectroscopy. From the valence-orbital spectra we are able to distinguish three different phases of adsorbed ${\mathrm{H}}_2$O: (a) single-adsorbed ${\mathrm{H}}_2$O molecules at temperatures close to the desorption point, (b) partially hydrogen-bonded ${\mathrm{H}}_2$O clusters for coverages of a monolayer or less, and (c) fully hydrogen-bonded ice at low temperatures and several monolayers of coverage. For phase (a), we find valence molecular orbitals which are almost rigidly shifted upwards relative to the gas phase by a final-state relaxation shift of 1.3 eV. All orbitals are broadened by 1.0-1.5 eV relative to the gas phase. For phase (b), we identify two inequivalent types of ${\mathrm{H}}_2$O molecules whose orbital energies differ by 1.5-2 eV. This splitting is identical to the electrostatic shift of molecular-orbital energies as calculated for the hydrogen-bonded ${\mathrm{H}}_2$O dimer by Umeyama and Morokuma. In this model the set of molecular orbitals with higher binding energy is assigned to the hydrogen-acceptor molecule and the set with lower binding energy to the hydrogen-donor molecule. At monolayer coverage we find about twice as many donors as acceptors.

• Received 28 June 1982

DOI:https://doi.org/10.1103/PhysRevB.27.3279