Multimode vibronic coupling effects in molecules

The multimode vibronic coupling problem is discussed and compared with the standard single-mode problem. It is demonstrated for two typical examples that the simultaneous consideration of two vibrational modes is essential to explain the experimental findings. The coupling of the involved electronic states via these modes gives rise to a conical intersection of the potential energy surfaces. The presence of the conical intersection strengthens considerably the nonadiabatic effects in the nuclear motion. Conical intersection of potential energy surfaces constitute a widespread phenomenon which should be considered when investigating vibronic coupling effects in realistic polyatomic molecules.