2-PI-RESONANCE BROADENING IN X-RAY-ABSORPTION SPECTROSCOPY OF ADSORBED CO

A systematic study of the broadening of the C 1s and O 1s 2pi resonances for CO adsorbed in different ordered phases on Ni(100), Pd(100), and Cu(100), and for CO on Ag(110) is presented. The systems span a number of different adsorption energies and adsorption sites, as well as different adsorbate-adsorbate distances. The widths of the 2pi resonances are found to be larger for strongly bonded and highly coordinated adsorbates but they are independent of the adsorbate-adsorbate separation. The broadening is larger for the O 1s than the C 1s 2pi resonances and it increases with the final-state chemisorption energy. For weakly chemisorbed final states, the 2pi width is dominated by vibrational excitations, while the adsorbate-substrate hybridization of the final core excited state determines the width in the case of strong chemisorption. Low-energy vibrational modes, like frustrated translations which are important for the broadening of adsorbate core-level photoelectron spectra, do not give any dominating contribution to the 2pi-resonance broadening.