Franck-Condon breakdown in core-level photoelectron spectroscopy of chemisorbed CO

The photon energy dependence of the vibrational fine structure in the Cls a nd Ols X-ray photoelectron main lines of chemisorbed CO on Ni(100) and Ru(0 001) has been measured from 6 to 150 eV above the core-level thresholds. Si gnificant deviations from the behavior in gas-phase CO are found. A strong dominance of the adiabatic peak towards threshold is found for the Cls, but not the Ols, lines. In the Cls lines, we observe a broad maximum of vibrat ional excitation 5 eV above the shape resonance. At high photon energies, F ranck-Condon behavior is observed in both the Cls and Ols lines. This behav ior is discussed in terms of the adsorbate electronic structure and the dyn amic metallic screening upon core ionization. (C) 1999 Elsevier Science B.V . All rights reserved.