Resonant photoemission from TiO2(110) surfaces: Implications on surface bonding and hybridization

Valence band photoemission obtained near the Ti 3p absorption edge has been used to investigate the surface electronic structure of TiO2(110)-(1x1),-(1x2) and Na/TiO2(110). The variation of the energy distribution curves with photon energy suggests that the observed resonant emission should be divided into two parts. A low-energy part is attributed to Ti 3p-->3d initial exitations, while a high-energy part is assigned to Ti 3p-->4s exitations. We find that both the intensity and the shape of the resonance profile as well as the spectral shape of the resonant emission vary substantially with surface conditions, the largest changes being associated with Ti 4s initial excitations. By applying a molecular orbital bonding scheme developed for rutile compounds and by considering the surface-induced perturbations to this, we are able to relate our experimental findings to recent structural models for the (1x1) and (1x2) surfaces. Particularly for the (1x1) surface the existence of bonding Ti 4s-O 2p hybridization observed at E(B)=5.6 and E(B)=6.7 eV is recognized for the first time. The observed restriction of this resonant emission to the clean (1x1) surface is readily associated with the contraction of Ti(S-fold)-O bond lengths expected for this surface. For the (1x2) surface we observe a strong damping of the 4s-related bonding resonances, a decrease in 3d-bonding emission and an increase of the 3d-''non-bonding'' emission. These observations are in accordance with a recent structural model for this surface. Finally, the resonant-emission data demonstrate that the surface Ti present at the (1x2) surface should be considered as Ti4+ states.