Anomalous surface core-level shifts in the hydrogen-induced reconstruction of W{100}

Shifts are reported in the W 4f(7/2), 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 theta(H) approximate to 0.33 and theta(H) approximate to 0.53, amounting to a shift to higher binding energy in the clean-surface core level of as much as 430 +/- 50 meV. By comparison with data from W{100} at saturation (theta(H) = 2), where the shift is only similar to 100 meV to higher energy compared to the clean surface core level, and data for H adsorption on other crystal planes, the large shift is mostly attributed to the pinching of pairs of surface W atoms induced by H adsorption at low coverages. The shift of similar to 370 +/- 50 meV associated with this reconstruction alone, to the exclusion of the influence of the adsorbate, is 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 is concluded that the anomalous shift is 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 theta(H) approximate to 0.33 and 0.53) provides evidences for the nature of both commensurate and incommensurate phases formed at the surface.