Vibrationally resolved C 1s photoemission from CO absorbed on Rh(1 1 1): the investigation of a new chemically shifted C 1s component

High-resolution core-level photoemission at an energy resolution better than the intrinsic width of the C 1s level has been used to study the CO/Rh(1 1 1) overlayer system. C 1s spectra have been measured in a large CO coverage range; from low coverages, where the CO molecules only adsorb in on-top positions, to saturation coverage, where CO molecules occupy both on-top and threefold hollow sites. Fine structure components due to vibrational excitation of the C–O stretch mode are clearly resolved in the emission peak from each site. The vibrational splittings and intensity ratios are found to be different for the different adsorption sites. A third C 1s component, which has not been resolved earlier, is found at intermediate CO coverages. Some possible explanations for the origin of this extra component are discussed. In the light of its binding energy and vibrational energy in photoemission, together with supporting evidence from high-resolution electron energy loss spectroscopy data and photoelectron diffraction, this is assigned to bridging CO species at intermediate coverages.