Vibronic coupling in molecular excited states

The influence of resonance and non-resonance vibronic coupling of two excited molecular states of a polyatomic molecule on the absorption and emission spectrum is discussed in terms of a non-adiabatic model (first introduced by Fulton and Gouterman) and in terms of the crude (CBO) and the adiabatic Born-Oppenheimer (ABO) approximation. It is assumed that static coupling of the CBO is due to an antisymmetric coupling term as a function of a single non-totally symmetric mode. Neither the CBO nor the ABO state is a good basis in general for the whole range of the coupling term for a perturbational treatment of the non-adiabatic interaction. Limits of different model approaches like harmonic ABO potentials and a linear coupling term are discussed. Numerical calculations for different model situations were performed.