Formula omitted] dispersed fluorescence spectra of glyoxal cooled in a supersonic jet: Energy and assignment of 264 S0 vibrational levels

Abstract

We have observed, measured, and assigned 264 vibrational levels of the electronic ground state S 0( X ̃ 1A g) of glyoxal (C 2H 2O 2) by studying the dispersed fluorescence spectra of the molecule excited by a cw dye laser. The cooling effect of a supersonic expansion simplifies considerably the excitation spectrum and allows us to excite the rotationless level of about 40 different (and previously assigned) vibrational levels of the excited electronic state S 1( A ̃ 1A u) . The dispersed fluorescence spectrum from each of the excited levels displays from about 10 up to 140 S 0 vibrational levels of a g and b g symmetry only. About 2000 S 1( V′) → S 0( V″) vibrational transitions have been observed. The energies of 335 S 0 vibrational levels have been measured and, among these, 264 have been assigned. Uncertainties in vibrational energies (without rotation) range from 0.15 to about 2 cm −1 according to resolution and signal to noise. Overtones with up to 14 quanta (in the torsional mode) and combinations involving up to four modes have been observed. As most of the observed levels are combinations, we have extensive information about the coupling between modes. Most of the energies are well fitted with a Dunham expansion: we have determined 55 of the 78 x ij parameters as well as some higher-order terms. Vibrational levels with energies up to 5000 cm −1 were definitively assigned even though the vibrational level density per symmetry becomes as large as 2 levels/cm −1. Furthermore, very few Fermi resonances have been observed. We claim that for no other molecule are so many vibrational energy levels known in a single electronic state.