Quenching and restoring of the A 2PI cationic state in resonant Auger electron spectra of CO in the vicinity of the O 1s-->2pi resonance

The evolution of the vibrational intensity distribution of the singly ionized A 2PI state in CO is experimentally examined for photon energy detunings below the adiabatic 0-0 transition of the O 1s-->2pi resonance. We have found a strong suppression of the entire vibrational fine structure of this state, leading to its almost complete quenching for certain excitation energies, followed by a partial restoring for larger values of negative photon energy detuning. Our observation, that cannot be rationalized by the known model of a vibrational collapse for energy detuning, may be explained in terms of a Fano interference between the direct and resonant photoionization channels in the presence of strong lifetime vibrational interference.