The 2-photon photo-emission spectra of (110) surfaces were measured for the nearly perfect surface, and for surfaces modified by the introduction of defects and adsorbed molecules. Defects were generated on nearly perfect surfaces by using 3 methods: electron irradiation, annealing in vacuum, and Ar+ sputtering. Nearly perfect or damaged surfaces could be further modified by adsorption of O2 or H2O molecules. Two-photon photo-emission spectroscopy was used to investigate systematically the work function change due to the presence of defects or adsorbates. The 2-photon photo-emission spectroscopy detected both surface and bulk O vacancy defects. It was found, from results on O adsorption, that O vacancies created by electron irradiation were localized on the surface and could be removed by O2 adsorption at 100K. The surface defects were substantially different to those created by annealing or by ion sputtering, where vacancies in the sub-surface region were proposed. It was found that O2 acted as an acceptor molecule on surface defect states whereas H2O acted as a donor molecule. From simulation of the work function change as a function of dosage, the dipole moment of H2O adsorbed on the TiO2 surface was derived to be 0.5D positive outward. An unoccupied electronic state, 2.45eV above the Fermi level, was also found that appeared at sub-monolayer coverage of H2O. This was tentatively attributed to charge transfer from surface Ti to surface-adsorbed H2O molecules or OH ligands.

Two-Photon Photo-Emission Spectroscopy of TiO2 (110) Surfaces Modified by Defects and O2 or H2O Adsorbates. Onda, K., Li, B., Petek, H.: Physical Review B, 2004, 70[4], 045415