Bonding and adsorbate core level shifts of transition metal atoms on the Al(100) surface from density functional calculations

The core level shifts of transition metal (TM) atoms adsorbed on a cluster model of the Al(100) surface have been studied by density functional calculations. The adsorbate bonding is covalent and implies a partial rehybridization on the adatom. The calculated core level shifts do not correlate with the amount of charge transfer to or from the surface, but exhibit an excellent linear correlation with the d occupation of the TM atoms. Furthermore, we identify the auetration of the adsorbate orbitals into the (nuclear) potential of the surface as an effect that leads to a net stabilization of the core levels.