Valence-band structure of TiO2 along the Γ-Δ-X and Γ-M directions

Angle-resolved photoemission spectra of the (100) and (110) faces of TiO2 have been recorded in the photon-energy range from 18 to 47 eV. These normal emission data have been analyzed on the basis of direct transitions into free-electron-like final states, yielding valence-band dispersion relations, E(k), along the high-symmetry Δ and lines of the bulk Brillouin zone. Polarization selection rules are derived for the appropriate nonsymmorphic space group, which are used to identify the symmetries of the valence bands. Empirical dispersion relations are compared with the results of a band-structure calculation, which employed an ab initio atomic orbital-based method. While there is reasonable agreement overall, for a Δ-line empirical band at about 1.2 eV below the valence-band maximum (Evbm) there is no counterpart in the calculated bulk band structure. This discrepancy may be related to a TiO2(100) surface resonance predicted to lie 1.5 eV below Evbm.