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

Abstract

(3 + 1) and (3 + 2) Resonantly EnhancedMultiphoton (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 disentanglement of the vibrational structure of the 3s Rydberg transition from the V ← N quasi-continuum, and a 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 (3dπ y , 4dπ y , 5dπ y ) 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. †Permanent address: Atomic Energy Commission of Syria, PO BOX 6091, Damascus, Syria.