Electronic Doppler effect in resonant Auger decay of CO molecules upon excitation near a shake-up Pi resonance

We present an experimental observation of the electronic Doppler effect in resonant Auger spectra upon core excitation slightly above the carbon K edge of the CO molecule. Thus the electronic Doppler effect has been identified in above-threshold excitation, and in a transition of Pi symmetry. Ab initio calculations of the potential energy curves of the relevant states of CO and the wave packet technique have been employed to provide a theoretical background to the experimental studies. The weak feature around 299.4 eV in the photoabsorption spectrum, whose decay has been investigated by the present experiment, is assigned to double (core-valence) excitations to C 1s shake-up states vertical bar 1s(C)(-1)1 pi(-1)pi(*2)> with a strong dissociative character, and the Doppler splitting of the atomic peak has been reproduced by the simulation.