Changes to the occupied and unoccupied electronic structure of TiO₂(100) upon alkali metal reduction

Abstract
Direct and inverse ultraviolet photoemission spectroscopy (UPS and IPS) are used to measure the occupied and unoccupied electronic structure of TiO₂(100)1x1 and TiO₂(100)c(2x2)K. UPS results from stoichiometric TiO₂(100)1x1 are in good agreement with earlier studies. IPS data recorded from TiO₂(100)1x1 show two distinct features with energy positions 1.7 eV and 5.3 eV above the Fermi level (E_F). These features are associated with the Ti 3d derived t_2g and e_g levels. Upon formation of TiO₂(100)c(2x2)K, achieved by annealing K treated TiO₂(100)1x1, several changes are observed to occur to the surface electronic structure. Population of a band gap state pins E_F and removes band bending at the surface. The valence band of TiO₂(100)c(2x2)K is noted to be substantially narrower (of the order of 0.5 eV) than that of TiO₂(100)1x1. In addition to this, a sharp feature lying to the high binding energy side of the valence band is populated. Concurrently, intensity in the Ti t_2g derived conduction band level is lost. This redistribution of spectral intensity observed between TiO₂(100)1x1 and TiO₂(100)c(2x2)K is interpreted in terms of a K induced redistribution of charge at the surface. We use resonant photoemission to test the idea that this charge redistribution involves mainly the Ti d levels.