Nonradiative decay processes in benzene

Abstract

We investigate the behavior of single vibronic level nonradiative decay rates in benzene and benzene- d 6. The effects of excitation in a promoting mode which undergoes frequency and geometry changes in the S 1 relaxation (to T 1 or S 0) are considered in detail. Calculated relative nonradiative decay rates are compared with experimental values and are used to assign triplet state vibrational frequencies to the ν s, ν 10 and ν 16 vibrations. This comparison also indicates that none of these modes, nor the modes ν 1 and ν 6, are likely to be the dominant promoting modes for the S 1 → T 1 decay. Some simple expressions are given which provide good estimates of the vibronic state dependence of the non-radiative decay rates. In conjuction with experimental decay rate data, these estimates can aid in guiding spectral assignments of vibronic bands. Simple but general theoretical criteria are derived which are useful in determining those vibrations which are poor (or good) accepting modes. Our previous theory is generalized to consider absolute nonradiative decay rates. The results are used to suggest a possible mechanism for the “channel three” decay process observed by Callomon . Although the numerical applications presented here are to benzene electronic relaxation processes, the theoretical developments also apply to and the calcultions illustrate general features of nonradiative decay in the statistical limit.