Excess energy dependence in radiationless transitions. Triplet to ground state nonradiative rate calculations in benzene

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Abstract
The T1→S0 intersystem crossing (ISC) rates in benzene are calculated on the assumption that all T1 vibrational states with the same amount of excess energy are equally populated owing to fast intramolecular vibrational redistribution. The results are compared with the experimentally observed tendency that the ISC rate increases rapidly with excess energy and levels off at higher excess energy. The above tendency is seen when vibrational frequencies in T1 are assumed to be the same as in S1. Whether the rapid initial increase appears or not depends on parameters involved in the radiationless process; on the other hand, the saturation of the ISC rate in the high energy range occurs regardless of the parameters. Furthermore, the saturation proves to correlate with the fact that the slope of the density of states in a logarithmic scale becomes smaller as the energy increases.