Dynamics around a multidimensional conical intersection: A mixed quantum-classical model

We investigate the dynamics around a two-state conical intersection in a polyatomic molecule. The simplified model considered is an extension of that proposed and tested previously for the case of one active (coupling) mode and one passive mode. Here we study a multidimensional conical intersection built adding many other passive modes, which globally share a small fraction (up to 20%) of the total vibrational energy. The model simply consists in treating the coupling mode quantum mechanically, while the others are described as classical oscillators running unperturbed trajectories on the initial diabatic surface. The computed adiabatic transition probabilities show, in some conditions, a marked effect of the extra passive modes, which can be explained in terms of the time spent by the classical trajectories in the region where the diabatic coupling is at its maximum. (C) 1997 American Institute of Physics.