Excited states of C2H4 studied by (3+1) and (3+2) REMPI spectroscopy: disentangling the lowest Rydberg series from the strong π-π* V←N transition
Résumé
(3+1) and (3+2) Resonantly Enhanced Multiphoton (REMPI) spectroscopy has been carried out to study the Rydberg states of C2H4 in the region 55000-83000 cm-1. Differences and similarities were observed between these three-photon spectra and the one-photon absorption spectrum. First, the disappearance of the strong π-π* V←N valence transition from the REMPI spectra allowed to disentangle the vibrational structure of the 3s Rydberg transition from the V←N quasi-continuum, and to search for additional weak transitions. Earlier vibrational assignments from the absorption spectrum have been confirmed in the REMPI analysis of the 3s←N and ns,nd←N transition systems, although with very different band intensities. This analysis has provided additional weak band assignments involving the ν3 mode in the observed Rydberg transitions. New electronic components (3dπy, 4dπy, 5dπy) of the nd complex which are three-photon allowed but one-photon forbidden, have been tentatively assigned. The photoelectron (PES) spectra of the 3s and 3dσ lowest vibrational levels have revealed a deviation from a pure Rydberg character, in apparent contradiction with previous (2+1) REMPI-PES data involving gerade vibronic levels of the 3s Rydberg state, which led to a prominent Rydberg character for this state. Rydberg-valence and Rydberg-Rydberg vibronic interactions mediated via non-symmetric modes could explain the different behaviour between gerade and ungerade vibronic levels of the ethylene Rydberg states.
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