Vibrationally resolved C 1s photoemission from CO absorbed on Rh(111): theinvestigation of a new chemically shifted C 1s component

High-resolution core-level photoemission at an energy resolution better tha n the intrinsic width of the C Is level has been used to study the CO/Rh(l 1 1) overlayer system. C Is spectra have been measured in a large CO covera ge range; from low coverages, where the CO molecules only adsorb in on-top positions, to saturation coverage, where CO molecules occupy both on-top an d threefold hollow sites. Fine structure components due to vibrational exci tation of the C-O stretch mode are clearly resolved in the emission peak fr om each site. The vibrational splittings and intensity ratios are found to be different for the different adsorption sites. A third C Is component, wh ich has not been resolved earlier, is found at intermediate CO coverages. S ome possible explanations for the origin of this extra component are discus sed. In the light of its binding energy and vibrational energy in photoemis sion, together with supporting evidence from high-resolution electron energ y loss spectroscopy data and photoelectron diffraction, this is assigned to bridging CO species at intermediate coverages. (C) 2001 Elsevier Science B Y. All rights reserved.