Core-level spectroscopy of clean and adsorbate-covered Ta(100)

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Abstract
We present a theoretical and experimental study of the surface core-level shifts on Ta(100), clean and H- or O-covered. For the clean surface, the observed core-level shift is very large for surface atoms (740 meV) whereas it remains small for the first-underlayer atoms (140 meV). This is in complete agreement with our theoretical predictions. In the presence of adsorbates, the calculations give the right order of magnitude of the shifts, if we assume that O and H atoms adsorb at centered sites on Ta(100). This last result is surprising since it is well known that H atoms occupy a bridge site on W(100).