FAST AND SLOW SCREENING EFFECTS IN INVERSE-PHOTOEMISSION SPECTRA OF CO CU(110)/

Since the beginning of the implementation of inverse photoemission spe ctroscopy (IPES or BIS) in the investigations of unoccupied electronic levels of adsorbates, a question has been aroused on the role which i nitial state (chemical) and final state (dynamic screening) effects ma y play in the corresponding IPE spectra. Due to a sudden injection of the IPE electron into a localized level of an initially neutral adsorb ate the coupling of the electron charge to the substrate surface diele ctic response should give rise to a manifestation of strong final stat e effects in the measured spectral features. To asses this problem it is necessary to correlate the time scales characterizing various tempo ral components of surface screening with the lifetime of the electron in the adsorbate level investigated. In the case of adsorbate valence levels broadened into resonances by the hybridization with the substra te, one can separate the surface response into a ''fast'' and a ''slow '' component, relative to the lifetime of the resonance level. Followi ng this, we have calculated the effects of screening on the IPE spectr a of the 2pi-affinity level of the CO/Cu(110) adsorption complex usin g a time dependent approach. A good agreement of our results with the available experimental data confirms our assertion about possible mani festations of an interplay between the initial and final state effects in the IPE spectra of this system.