SURFACE CORE-LEVEL SHIFTS AS A PROBE OF THE LOCAL OVERLAYER STRUCTURE - CO ON PD(100)

The binding energies of the Pd 3d and the C 1s core levels have been studied for the p(2 square-root 2 x square-root 2) R45-degrees, p(3 square-root 2 x square-root 2) R45-degrees, and p(4 square-root 2 x square-root 2) R45-degrees structures of CO on Pd(100). Adsorption of CO shifts the Pd 3d surface emission to higher binding energies with the magnitude of the shift depending almost linearly on the number of Pd to CO bonds; the measured shift per bond is close to 0.5 eV. A direct relation is established between the detailed geometry of the CO overlayer and the changes in the Pd 3d spectra.