ON THE ADSORPTION SITES FOR CO ON THE RH(111) SINGLE-CRYSTAL SURFACE

High resolution photoemission applied to the C 1s and Rh 3d core level s has been used to investigate the adsorption sites at low temperature of CO on the Rh(111) single crystal surface as a function of CO cover age. Two different sites are found to be occupied by the CO molecules. At coverages up to 0.5 monolayers the majority of the molecules are f ound to adsorb in on-top sites whereas at higher coverages three-fold hollow sites become increasingly populated. The different C 1s binding energy of the CO molecules in these two sites makes it possible to me asure diffraction induced intensity variations versus photon energy in a site specific manner. The saturation (2 x 2)-3CO structure formed a t a coverage of 0.75 monolayers is argued to contain one on-top and tw o three-fold hollow molecules per unit cell.