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 W4f 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 (1x1) terrace areas where all the W ions lose on-top oxygen. © 1998 Elsevier Science B.V.
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