Vibrationally resolved resonant Auger spectroscopy of formaldehyde at the C 1s-1π* resonance

Vibrationally resolved resonant Auger spectra of gaseous formaldehyde have been measured under resonant Raman conditions at several photon energies on the C 1s-1π* resonance. Specific vibrational states of the C 1s-1π* resonance were selectively populated by utilizing high-resolution photoexcitation. Participator Auger decay of the excited state populates mainly the A-D ionic states, without any discernable increase in the intensity of the X state. The vibrational manifold of the A state band was examined in detail for non-resonant and resonant photoionization. While the ν2 vibrational mode was found to account for most of the structure in the non-resonant spectrum, strong excitations of the ν1 mode were found in the resonant spectrum with decreased intensity in the higher ν2 vibrations. Intensity distribution among the ν2 levels of the ionic A and B states in the resonant Auger decay were found to reproduce the ν2 level accessed in the photoabsorption. Both of these observations were attributed to changes in the potential energy surfaces of the core excited and ionic states, namely, lengthening of the C=O bond in the core excited and ionic states relative to the ground electronic state of the molecule, and shortening of the C-H bond in the excited state only