Cation-ligand hybridization for stoichiometric and reducedTiO2(110) surfaces determined by resonant photoemission

We have performed a resonant photoemission study on both stoichiometric and reduced ${\mathrm{TiO}}_2$ (110) surfaces using synchrotron radiation. A resonant enhancement of the O 2p valence-band photoemission intensity is observed for both stoichiometric and reduced ${\mathrm{TiO}}_2$ surfaces as the photon energy is swept through the Ti 3p→3d optical transition energy, which indicates a strong hybridization between the oxygen and titanium ions. Compared to the stoichiometric surface, the amplitude of the resonance is smaller for the reduced surface. In addition, the reduced surface has higher emission intensity from the O 2p bonding orbitals and lower emission intensity from the nonbonding orbitals. For both stoichiometric and reduced surfaces, the intensity of emission from the O 2p bonding orbitals resonates more than that from the O 2p nonbonding orbitals, and the resonance profile for the nonbonding orbitals maximizes at a higher photon energy than that for the bonding orbitals. The resonant enhancement of the O 2p bonding orbitals is consistent with theoretical calculations, which predict that the O 2p–Ti 3d hybridization involves primarily those O 2p orbitals. The energy dependence of the resonant enhancement for the O 2p nonbonding orbitals suggests that it results predominantly from resonant processes involving Ti 3p→4s excitation at the surface.