Radiationless decay of vibronically coupled electronic states. II. Non-Condon effects of nontotally symmetric accepting modes

Radiationless transitions from an optically prepared state to the ground state are studied on a model consisting of three electronic states and three harmonic modes of vibration. One nontotally symmetric mode (the inducing mode) couples the lower excited state to the ground state, another, of different symmetry (the coupling mode) couples the two excited states, and the third mode which is totally symmetric is not involved in vibronic coupling but acts only as an energy-accepting mode. If the vibronic couplings are weak, the system can be solved by the methods of Part I [J. Chem. Phys. 69, 5496 (1978)]. The properties of this model are compared with those of a more general model that permits anharmonic adiabatic potentials. If the energy separation between the excited states is small, the upper excited state affects the decay properties of the lower one through two mechanisms, namely, (i) a frequency change in the coupling mode which improves its efficiency as an accepting mode and (ii) a non-Condon effect of the coupling mode on the transition matrix element of the inducing mode. Mechanism (i) corresponds to the proximity effect discussed by Lim et al., but mechanism (ii) which tends to dominate for small energy separations of the excited states and large energy gaps to the ground state, has not been discussed before. The experimental implications of the latter mechanism are briefly discussed.