Theory of delayed molecular fluorescence

In this paper we present a quantum mechanical model for the phenomenon of delayed, temperature dependent, fluorescence from a polyatomic molecule in a dense medium or in the high-pressure gas phase. We have handled the radiative and the nonradiative decay processes of a manifold of scrambled vibronic levels corresponding to electronically excited singlet and triplet (zero-order) configurations, where the molecule-medium coupling provides the mechanism for thermal population of the decaying resonances. Explicit expressions were derived for the temperature dependent decay lifetime and for the emission quantum yields. The quantum mechanical rates could be expressed in terms of the decay widths of the zero-order Born-Oppenheimer states, providing a proper justification for the phenomenological three-level kinetic model which was conventionally utilized for this problem. New results were obtained for the temperature dependence of the decay rate, relating microscopic dynamic information, obtained from optical selection studies on "isolated" molecule, to macroscopic kinetic data for the molecule in a dense medium. Numerical results for the temperature dependence of the decay characteristics of medium-perturbed biacetyl are in reasonable agreement with the available experimental data.