Resonant Two-Photon Photoemission from Ti 3d Defect States of TiO2(110) Revisited

Abstract

Recently three groups reported on resonant two-photon photoemission (2PP) from Ti 3d defect states on rutile TiO2(110) surface. The first publication (Argondizzo, A.; Cui, X.; Wang, C.; Sun, H.; Shang, H.; Zhao, J.; Petek, H. Phys. Rev. B 2015, 91, 155429) to appear assigned the resonance enhancement of the two-photon absorption processes to excitations from the nominally t2g symmetry Ti3+ 3d defect states of reduced bulk TiO2 through a one-photon resonance with nearly degenerate pair of eg symmetry intermediate states, before absorbing another photon into the photoemission continuum. This resonance occurs for hν ∼ 3.6 eV, and has pronounced dependence on the alignment of the electric field vector of the excitation light with the in-plane crystalline axes of the TiO2(110) surface. Subsequently, two other groups reported similar 2PP spectra of reduced TiO2 surfaces, with one claiming a single d–d transition of OH terminated TiO2 at a lower energy than in the primary report (Wang, Z.; Wen, B.; Hao, Q.; Liu,...