EXCITATION-ENERGY-DEPENDENT RESONANT PHOTOEMISSION - C1S-PI-ASTERISK SPECTRA OF CARBON-MONOXIDE

High-resolution measurements of the x-ray resonant photoemission of CO near the C1s-->pi transition are presented. The branching ratios for the valence shell ionization close to the resonance how a strong depe ndence on the photon energy with a clear asymmetry around the resonanc e energy. It is shown that this energy dependence is due to the presen ce of the direct photoionization channel in addition to the resonant c hannel and that it can be theoretically described in the framework of resonant multichannel ionization by a Fano approach. From this general formulation two models, including or excluding the interference betwe en the direct and resonant processes, are derived and numerically eval uated. The no-interference model can explain only the gross features o f the measured branching ratios, while inclusion of interference terms in the model can reproduce. almost quantitatively, the experimental r esults and justifies the observed asymmetry of the branching ratios. I t is also proved that the lifetime vibrational interference is absent in the cross section integrated over the photoelectron energy. [S1050- 2947(97)01412-1].