Electronic states associated with oxygen deficiency at WO3(001) surfaces have been studied by photoemission with the photon energy tuned to a resonance maximum in the W 5d ionization cross-section profile. Mild ion bombardment followed by UHV annealing leads to a reduced surface where the predominant W 5d states lie “deep” within the bulk bandgap. After more energetic and prolonged bombardment and annealing, a quasi metallic surface is obtained with a significant density of states at the Fermi energy. The valence level photoemission measurements are compared with results from scanning tunnelling microscopy and shallow W 4f core level photoemission. The “deep” states are associated with pairs of W ions σ-bonded together in defect troughs that can be imaged in real space by STM. By contrast the metallic states are assigned to reduced (1 × 1) terrace areas where all the W ions lose on-top oxygen.
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