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The fluorescence lifetimes of, and quantum yields of emission from, individual vibronic levels of isolated molecules of styrene, trans-β-styrene-d1, styrene-d8, and ethynlbenzene have been measured. These data are subjected to a decomposition, under a plausible hypothesis, into parameters descriptive of an isomerization process and an intersystem crossing process. It is argued that the two processes proceed independently in parallel. The rate of isomerization is found to be independent of the vibrational energy content of the isolated molecule and to have a dependence on isotopic mass consistent with the prediction that the torsional mode is the best ground state acceptor in the reaction channel. A plausible schematic potential energy surface is constructed from a combination of spectroscopic observations and analogies with theoretically derived surfaces for similar molecules. Analysis of the experimental data, and deductions from those data, in terms of the properties of the potential surface, leads to the inference that intramolecular vibrational relaxation is slower than chemical reaction for excess vibrational energy up to ∼2500 cm−1.

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