CORE-LEVEL SHIFT SPECTROSCOPY FOR ADSORBATES - IONIC, COVALENT OR METALLIC BONDING

Shifts in the core-level binding energies of surface substrate atoms induced by adsorption have often been attributed solely to charge transfer between adsorbate and substrate atoms, leading to the recent conclusion that alkali-metal adatoms are non-ionically bonded. A more detailed analysis, including specific computations of the core-level shift-induced simply by altering the coordination of surface W atoms in W{110} by Cs and O adsorption, indicates that shifts due to this purely environmental effect, and those incurred by final state relaxation, are at least as big as those incurred by charge transfer. In particular, whereas for electronegative adsorbates all of these effects contribute towards a shift to higher core-level binding energies, for electropositive adsorbates the environmental shift towards higher binding energy may be cancelled by charge transfer and relaxation shifts towards lower binding energies.