Oxygen-induced surface core-level shift and angle-resolved X-ray photoemission spectroscopy of c(2 × 2)O/Pd(100)

Abstract

The combination of the surface core-level shift (SCLS) and angle-resolved X-ray photoemission spectroscopy (ARXPS) has been used to investigate the spatial extent of the oxygen-induced surface component for the Pd 3d 5/2 core level from c(2 × 2)O/Pd(100). X-ray photoelectron diffraction (XPD) patterns of this oxygen-induced surface component were dominated by forward focusing. Polar angle scans along the rarely studied 〈012〉 and 〈013〉 as well as the low-index 〈010〉 and 〈011〉 azimuths give a consistent picture of the location of the source atoms associated with these SCLS. The observed XPD patterns are compared with the results of a single scattering cluster (SSC) calculation, and the major forward focusing maxima along 〈012〉 and 〈013〉 azimuths have been identified. In addition, the observed angle dependence due to the loss of photoelectron intensity by inelastic scattering was successfully described using a uniform layer model together with an empirical instrument response function. Quantitative analysis using a theoretical model for surface states supports the idea that the oxygen-induced SCLS is a signature of extended surface states decaying exponentially into the bulk. These extended surface states indicate that “chemical” interactions may propagate beyond nearest neighbors inward, analogous to recent reports that chemisorption was influenced by buried metal substrates [J.A. Rodriguez, J. Phys. Chem. 98 (1994) 5758, and references therein].