THE PI ASTERISK RESONANCE AND ITS VIBRATIONAL BROADENING OF UNPERTURBED AND ADSORBED C2H4 MOLECULES

A refined NEXAFS analysis of ethylene physisorbed, chemisorbed and condensed on Cu(100) and Ag(100) will be given. Through deuteration we clearly identify C-H and C-C vibrations. Applying a simple linear coupling model the Frank-Condon factors for the core excitation process are derived. The only fit parameters are h-omega-BAR and one single intensity factor S for each vibrational progression, and one linewidth gamma for all lines. Form these, we determine the change in bond length for the molecules in the excited state and the decrease of the core hole lifetime upon increasing strength of chemisorption. From the change of the S factors upon chemisorption we determine the force constant that couples the molecule to the substrate. The approach of our analysis will be compared to the recent calculation by Gaeda et al.