Core-level shifts in bulk alloys and surface adlayers

The (initial state) core-level shifts of transition metals in bulk alloys and for bimetallic adlayer systems are calculated and analyzed in terms of extra- and (l-decomposed) intra-atomic contributions. Both the trends and magitudes of the calculated shifts are consistent with experimental data where they exist. Core-level shifts of the same sign for both constituents are common in both alloy and adlayer systems. The commonly used ‘‘charge transfer’’ model for interpreting core-level shifts is shown to be grossly inadequate, especially for describing the adlayer shifts; for bulk alloys, however, there is a correlation between the shifts and the changes in the d electron count. The results support the view that the bonding in bulk alloys and adlayers is fundamentally the same; in particular, there is no evidence for any anomalous charge transfer in adlayer systems. The extra-atomic contributions to the adlayer shifts are found to dominate the total shifts for Cu/Rh(001), Cu/Ta(110), and Pd/Ta(110), with the intra-atomic contributions often having the opposite sign compared to the total shift. For the substrate, however, the intra-atomic contributions are relatively more important, suggesting that substrate core-level shifts may provide a better measure of the local chemical effects than adlayer shifts. The relationship of core-level shifts to other physical properties is also discussed.