The vibrational level structure of trans-glyoxal in the T1 (3Au) state

Jet-cooled glyoxal molecules were excited on the T1(3Au)←S0(1Ag) transition. The frequencies of 62 bands in the T1→S0 phosphorescence excitation spectrum between 19 120 and 21 400 cm−1 were determined to within ±2 cm−1. 25 bands were assigned to eight normal modes, of which ν3, ν6 and ν8 were not known previously. The T1 and S1 frequencies agree on average to within 2%. All band profiles are of the `parallel bandʹ type except for the 801 band which is `perpendicularʹ and vibrationally symmetry-forbidden. Its relative intensity is 100 times smaller than that of the likewise forbidden S1←S0 801 band.