Excitation-energy-dependent resonant photoemission: C1s-π*spectra of carbon monoxide

High-resolution measurements of the x-ray resonant photoemission of CO near the C$1s\to {\pi }^{*}$ transition are presented. The branching ratios for the valence shell ionization close to the resonance show a strong dependence on the photon energy with a clear asymmetry around the resonance energy. It is shown that this energy dependence is due to the presence of the direct photoionization channel in addition to the resonant channel 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 between the direct and resonant processes, are derived and numerically evaluated. The no-interference model can explain only the gross features of the measured branching ratios, while inclusion of interference terms in the model can reproduce, almost quantitatively, the experimental results and justifies the observed asymmetry of the branching ratios. It is also proved that the lifetime vibrational interference is absent in the cross section integrated over the photoelectron energy.