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