Photophysical processes in the vapour phase measured by the opto-acoustic effect. Part 5.—Relaxation of highly vibrationally excited ground state benzene molecules

Abstract

Spectrophone data are presented for low pressures of benzene excited so as to produce the ground state with ≈ 40 000 cm^–1 excess vibrational energy. Excitation was to the benzene first excited singlet state using the 248.4 nm line from a Hg arc; at sufficiently low pressures this results in transfer, via the triplet manifold, to high vibrations in the ground state. These are compared with measurements using methane + benzene mixtures excited to the methane ν₃ level with a He–Ne laser at 3.39 µm and resulting in population of the C—H stretching vibrations of benzene at ≈3060 cm^–1. Models are developed for the relaxation of the large excess of vibrational energy either involving successive vibrational–translational steps or involving vibrational–vibrational processes followed by a final relatively slow vibrational–translational transition. The latter is shown to give much better agreement with experiment; several parameters associated with the vibrational–vibrational cascade are calculated and discussed.