“Hidden” vibrations in CO: Reinvestigation of resonant Auger decay for the C 1s⃗π^* excitation

We report an experimental investigation of higher vibrational levels (ν^′>2) of the C 1s⃗π^* excitation in CO utilizing resonant Auger electron spectroscopy. These vibrational states are not readily seen in a total yield absorption spectrum, whereas they are shown to be discernable in a partial electron yield spectrum. Furthermore, the nature of the spectator part between E_Bin=26.5 and 30 eV, being subject of an ongoing debate, is discussed on the grounds of the presented experimental data. The experiments indicate that the formally used adiabatic framework for the multitude of close lying ²Π potential curves is inappropriate, but that a diabatic treatment can be used for the interpretation of the spectra.