Effects of anharmonicities on radiationless transitions in large molecules

A new computational method to account for the effect of anharmonicities on non-radiative decay rates of large molecules in the statistical limit is given by using expansions of the generating functions which have been used successfully in multiphonon processes. This method is free from the ambiguity which, as in previous conventional methods, arises from the factorization which separates the decay rate into a product of an electronic term, a Franck-Condon factor and a density of final states. The rate expression is asymmetric with respect to the sign of the displacement of the accepting modes. Actual calculations are performed for a model molecule which closely resembles benzene. The numerical data for the decay rate exceed the results of the harmonic approximation by about two orders of magnitude.