Radiationless Transitions in Isolated Molecules. The Effects of Molecular Size and Radiation Bandwidth

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Abstract
Light absorption and emission characteristics of an isolated molecule are examined in terms of the stationary states of the molecule and the properties of the incident light (e.g., coherence and bandwidth). The use of various correlation functions and correlation times for the molecule and radiation field permits a detailed description of the time dependence of various excitation processes. The formalism developed, when applied to a simplified model, provides an explanation for the observed differences in fluorescence behavior between large and small molecules. The conditions of validity of the usual description of molecular excitation (followed by internal conversion) in terms of Born–Oppenheimer (B–O) states is discussed in terms of the molecular stationary states and radiation bandwidth. It is shown in terms of the model that if the radiation bandwidth is sufficiently large excitations occur to an electric‐dipole‐allowed B–O state and that photon emission is determined in a simple way by the relaxation function for that state. The distinction between large and small molecules depends on the density of final B–O states relative to their coupling strength with the allowed B–O state. Comparisons are made with the work of Jortner and Berry [J. Chem. Phys. 48, 2757 (1968)], who have studied this problem using a different formalism.

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