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.

Acknowledgments

First of all, two of the authors (DG and A-LR) are most grateful to Dr Ch. Jungen for having shared with them part of his thorough understanding of molecular Rydberg states. The authors wish to thank Dr S. Douin and Dr T. Pino for their help in the earlier experimental stage. They acknowledge financial support from the ‘Programme de Physique et Chimie du Milieu Interstellaire’ and from the ‘Programme National de Planétologie’ (CNRS). One of the authors (KA) acknowledges financial support from the Atomic Energy Commission of Syria.

Notes

†Permanent address: Atomic Energy Commission of Syria, PO BOX 6091, Damascus, Syria.