ELECTRONIC STATES AT OXYGEN-DEFICIENT WO3(001) SURFACES - A STUDY BY RESONANT PHOTOEMISSION

Electronic states associated with oxygen deficiency at WO3(001) surfac es 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 stares lie ''deep'' within the bulk bandgap . After more energetic and prolonged bombardment and annealing, a quas i metallic surface is obtained with a significant density of states at the Fermi energy. The valence level photoemission measurements are co mpared with results from scanning tunnelling microscopy and shallow W 4f core level photoemission. The ''deep'' states are associated with p airs of W ions sigma-bonded together in defect troughs that can be ima ged 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 oxyge n. (C) 1998 Elsevier Science B.V.