Optical selection studies of radiationless decay in an isolated large molecule

In this paper we present the results of a theoretical study of the nonradiative decay probability of a single vibronic level of a large isolated molecule. Utilizing Feynman's operator techniques, we were able to derive a theoretical expression of the dependence of the electronic relaxation rate on the excess vibrational energy in the excited electronic state for a "harmonic molecule" which is characterized by displaced potential surfaces. For a large effective electronic energy gap the nonradiative decay probability increases with increasing excess vibrational energy, while for a small energy gap the nonradiative decay in higher vibronic levels may be retarded. Our rough numerical calculations are found to be consistent with recent experimental data on optical selection studies in the isolated benzene molecule.