Radiationless decay of vibronically coupled electronic states. III. Strong coupling and its effect on triplet decay in aromatic hydrocarbons

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Abstract
Radiationless transitions from an optically prepared state to the ground state are studied on a model consisting of three electronic states and four modes of vibration. Two ’’promoting’’ modes induce transitions from the two excited states; they are taken harmonic and treated in the weak-coupling approximation. One mode couples the two excited states; its coupling is allowed to be strong enough to lead to anharmonic adiabatic potentials. The fourth mode is a displaced harmonic oscillator acting as an accepting mode, a role it shares with the coupling mode if the coupling is strong enough. The model differs from that studied in part II [J. Chem. Phys. 72, 1641 (1980)] in allowing stronger coupling as well as indirect (second-order) transition via the upper electronic state. Solutions are obtained by numerical methods that avoid the use of the Condon approximation. Radiationless decay rate constants are calculated for cold and hot bands and related to the energy distribution between the accepting modes. They are plotted as a function of energy separations, displacements, and coupling strengths. The results which confirm and extend the conclusions of part II are applied to radiationless triplet decay in benzene, naphthalene, and their perdeuterated analogs. It is concluded that in the two benzenes, in naphthalene-d8, but not in naphthalene-h8, the mode coupling T1(3La) with T(3Ba) is the dominant accepting mode. This result is used to account for anomalies in the deuterium effect and energy-gap dependence of triplet decay rate constants in aromatic hydrocarbons.