Core-level shifts and oxidation states of Ta and W: Electron spectroscopy for chemical analysis applied to surfaces

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Abstract
Surface-sensitive core-level photoelectron spectra are presented for surface oxide phases of Ta and W in the monolayer range. Discrete chemically shifted core levels are observed which are up to 5 times sharper than in bulk oxides. A simple model is proposed for explaining the wealth of corelevel shifts in terms of discrete oxidation states. Application of concepts from electron spectroscopy for chemical analysis yields a chemical shift of ≈0.42 eV per metal—oxygen bond for surface oxide phases. With this interpretation, Ta exhibits predominantly odd oxidation states (1,3,5) and W even oxidation states (2,4,6). In addition to the chemical shift, a relaxation shift of 2-3 eV has to be included for insulating oxides which accounts for the lack of metallic screening. Discrete configurations with characteristic core-level intensities and shifts are found in Ta.