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The fluorescence excitation and emission spectra of trans-β-methyl styrene have been measured in a supersonic jet. A complete vibrational assignment of the S0 and S1 states’ frequencies is reported, assisted by ab initio quantum chemical calculations and by comparison with the IR spectrum. The fluorescence lifetime, τf, of the isolated molecule changes monotonously from 24.5 to 15 ns as the excitation energy increases from the origin band to an excess of 3000 cm−1. The fluorescence quantum yield from the zero-point energy level of S1 is about 38%, similar to the liquid solution value; The major radiationless process being intersystem crossing to a triplet level. The increasing congestion of the emission spectra as the excitation energy is increased is interpreted as due to intramolecular vibrational energy redistribution. The data are consistent with the fact that in the isolated molecule intramolecular vibrational energy redistribution is faster than intersystem crossing. Beyond an excess energy of about 3200 cm−1, a more pronounced decrease in τf is observed, indicating that the barrier to trans–cis isomerization on the S1 surface, in the isolated molecule is higher than 3200 cm−1.

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