High-resolution core-and valence-level photoemission spectra of Nb-doped TiO 2 ceramics (Ti 1Ϫx Nb x O 2 with 0.01ϽxϽ0.8) have been measured using monochromatic x-ray excitation. Nb doping produces a welldefined photoemission peak in the bulk band gap of rutile, whose intensity increases with increasing doping level. Core-level spectroscopy shows that the Nb is incorporated within the rutile lattice at low doping levels mainly as Nb͑V͒ and that the gap state is associated with Ti͑III͒ ions. This conclusion is reinforced by variable energy photoemission measurements on Ti 0.9 Nb 0.1 O 2 in the vicinity of the Ti 3p and Nb 4p core thresholds. The photoemission resonance profile for the gap states reaches half maximum intensity at the same energy as found for oxygen-deficient TiO 2Ϫx but is shifted from the resonance profile for the Nb 4d states of NbO 2 . STM images on Nb-doped TiO 2 ͑110͒ are considered in relation to the spectroscopic measurements. Nb dopant atoms are imaged as ''bright spot'' clusters, implying delocalization of charge from Nb onto neighboring Ti ions. The experimental x-ray photoelectron spectroscopy data are compared with density-of-states profiles derived from local-density approximation calculations on pure and Nb-doped TiO 2 clusters. These calculations show that Nb doping of TiO 2 introduces new states of mixed Nb 4d -Ti 3d character above the O 2p valence band of the host material. In addition, there is increased x-ray photoemission intensity across the O 2p valence band owing to strong Nb 4d/O 2p hybridization and a cross section for ionization of Nb 4d states that is an order of magnitude larger than that for O 2p or Ti 3d states.

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It is found that an ordered and air-stable GaAs͑111͒A-͑1ϫ1͒-Cl surface can be produced by chemical etching/passivation with dilute HCl solution. The synchrotron polarization-dependent Cl K-edge x-ray absorption near-edge structure and x-ray photoelectron spectroscopy studies showed that the surface is terminated with Ga-Cl bonds oriented along the surface normal. Low-energy electron diffraction studies showed a bulklike ͑1ϫ1͒ structure on the Cl-terminated GaAs͑111͒A surface. The Cl termination eliminates surface band-gap states caused by surface oxides. Photoluminescence measurements showed a dramatic increase in the near-band radiative emission rate corresponding to reduction in the occupied surface band-gap states. A reduction of surface gap states by Cl termination was confirmed by surface photovoltage measurements.