Shifts were reported in the W 4f7/2 core-level binding energy of surface atoms involved in the hydrogen-induced restructuring of W {100}. A detailed analysis of the data reveals anomalously large shifts peaking at hydrogen fractional surface coverages θH ≈ 0.33 and θH ≈ 0.53, amounting to a shift to higher binding energy in the clean-surface core level of as much as 430meV. By comparison with data from W {100} at saturation (θH = 2), where the shift was only ≈100meV to higher energy compared to the clean surface core level, and data for H adsorption on other crystal planes, the large shift was mostly attributed to the pinching of pairs of surface W atoms induced by H adsorption at low coverages. The shift of ≈370meV associated with this reconstruction alone, to the exclusion of the influence of the adsorbate, was considerably larger than expected by comparison with extensive data for both the clean surface reconstruction on W {100} itself and on a range of other single-crystal planes of W. It was concluded that the anomalous shift was consistent with a hydrogen-induced inward displacement of the surface potential barrier, as deduced by Herlt and Bauer from electron reflection coefficient measurements. The coverage dependence of the core-level shifts (in particular their two maxima at θH ≈ 0.33 and 0.53) provides evidences for the nature of both commensurate and incommensurate phases formed at the surface.

Anomalous Surface Core-Level Shifts in the Hydrogen-Induced Reconstruction of W {100}. J.Jupille, K.G.Purcell, D.A.King: Surface Science, 1996, 367[2], 149-61