Intrinsic and extrinsic plasmon coupling in x-ray photoemission from core states of adsorbed atoms

The coupling of a surface plasmon to an adsorbate core level in x-ray photoelectron spectroscopy has been observed for the $1s$ level of oxygen adsorbed on an aluminum (111) surface. The coupling parameter was found to be strongly coverage dependent. To distinguish between intrinsic and extrinsic effects the dependence of the coupling parameter on the photoelectron escape angle was also measured. In this way both extrinsic and intrinsic terms could be identified, as well as the interference term. Within the framework of the model used for the interpretation of results, the intrinsic effect was found to be low in magnitude with a coupling parameter of \ensuremath{\sim}0.04, which implies a surface-plasmon screening energy of about 0.4 eV. This unexpectedly weak core-hole-plasmon coupling is explained by considering the repulsion of the screening charge by the valence electrons of the adsorbed atom.