Photoemission Studies of Molecular Adsorption on Oxide Surfaces
We have used ultraviolet photoemission spectroscopy (UPS), LEED and Auger spectroscopy to study the interaction of adsorbed molecules with transition-metal-oxide surfaces. Comparison of the UPS spectra of free molecules with the UPS difference spectra for adsorbed molecules, makes it possible to observe changes in the molecular electronic structure induced by adsorption. We have studied the adsorption of O2 and H2O on both nearly perfect and ion-beam-damaged surfaces of TiO2 and SrTiO3. The interaction of O2 with all surfaces is similar for low exposures (≲ 100 L), with the difference spectra indicating strong, probably dissociative chemisorption. For O2 exposures, ≳100 L, a second adsorbed phase occurs on some surfaces. For low H2O exposures, the difference spectra give evidence for dissociative chemisorption in some cases. For high H2O exposures, the spectra indicate only a slight distortion of the free molecule. On TiO2, the a1 molecular orbital of H2O is shifted 0.5—1eV toward tighter binding, suggesting that H2O is bound to the surface via is O lone-pair orbital. On SrTiO3, both the a1 and the higher-lying b1 molecular orbitals are shifted, suggesting a more complicated bonding. The magnitude of the extramolecular relaxation-polarization shift of H2O on adsorption depends primarily on the presence or absence of defect electronic surface states in the bulk bandgap of the oxide.
KeywordsValence Band Adsorbed Molecule Difference Spectrum Adsorbed Phase Free Molecule
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