(3+1) resonance enhanced multiphoton ionization photoelectron spectroscopy on nf Rydberg states of carbon dioxide

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Abstract
The (3+1) resonance enhanced multiphoton ionizationspectrum of carbon dioxide has been investigated for the (2Π g3/2,1/2)nfRydberg states. The excitation spectrum has been obtained for the n=4–13 members, while photoelectron spectra with a resolution of 15 meV have been measured for the n=4–7 members. The photoelectron spectra demonstrate that a delicate balance exists between direct and indirect ionization processes as evidenced by the large differences in vibrational activity observed in the spectra obtained via various members of the (2Π g3/2,1/2)nfRydberg series. Strong vibrational activity, which would be forbidden by simple selection rules, is found and interpreted to arise in part from vibronic coupling between various ionic states. Long vibrational progressions, mainly involving the symmetric stretch vibration of the X 2Π g state of the ion, indicate further severe deviations from Franck–Condon behavior. Two‐color excitation of the (2Π g3/2,1/2)nfRydberg states provides strong evidence that the superexcited states accessible at the four‐photon level have a profound influence on the vibrational progressions and distributions in the photoelectron spectra. Finally, resonances associated with multiphoton excitation of atomic carbon and oxygen have been observed. The processes which might be responsible for the occurrence of these atoms are discussed.