Chemisorption-induced4f-core-electron binding-energy shifts for surface atoms of W(111), W(100), and Ta(111)

Abstract

Hydrogen- and oxygen-induced chemical shifts have been resolved in Ta and W substrate $4f$ core levels by high-resolution synchrotron-radiation-excited photoemission spectroscopy. The surface levels shift continuously by typically 100 to 200 meV to higher binding energy for increasing hydrogen coverage. This is due to a quenching of surface states in the valence band and a charge transfer from the substrate to the hydrogen atoms. Upon adsorption of oxygen, new core peaks appear at 0.4 to 1.3 eV higher binding energy with respect to the bulk level, indicating the formation of strong chemisorption bonds. Substrate chemical shifts for element $Z$ correlate with chemisorption-induced changes in the heat of surface segregation of the ($Z+1\mathrm{}$) constituent in a dilute ${\mathrm{}(Z+1\mathrm{})\mathrm{}}_x{Z}_{1-x}$ alloy.