Study of the electronic structure of TiO2(110) and Cs/TiO2(110) with metastable impact electron spectroscopy and ultraviolet photoemission spectroscopy(HeI)

We report the metastable impact electron spectroscopy (MIES) (He*1s2s) and ultraviolet photoemission spectroscopy (UPS) (HeI) spectra for clean TiO2(110) and Cs/TiO2(110). The cesium coverage is varied at room temperature between zero and saturation. Simulations of the MIES spectra for clean and cesiated TiO2(110), based on presently available information on the electronic structure of the clean and cesiated surfaces, facilitate the interpretation of the spectra. The MIES results for low Cs coverages are compatible with fully ionic adsorption of Cs whereby the 6s electron of Cs is transferred to the substrate. This causes reduction of Ti4+ cations as detected with UPS. For larger coverages the charge transfer is incomplete as judged from the formation of temporary negative He-*(1s2s(2)) ions during the interaction of the metastable He atoms with the cesiated surface. The ionicity, i.e. the charge transfer per adsorbed Cs atom, decreases from practically unity to about 12% at saturation. At saturation a rearrangement of the oxygen in the surface layer takes place with time. The results suggest that the Ti and Cs ions compete for the bonding to the oxygen. (C) 2000 Elsevier Science B.V. All rights reserved.