Quantum dynamics of photoexcited cyclohexadiene introducing reactive coordinates

The ultrafast photochemical ring opening reaction of 1,3-cyclohexadiene is investigated theoretically. To allow for a quantum mechanical treatment of this high-dimensional problem we introduce reactive coordinates which conta in the modes active on the femtosecond time scale. A reduced Hamiltonian is derived in the corresponding subspace and two-dimensional potential energy surfaces are interpolated using ab initio data points. The quantum dynamic s in the S-1 state toward two conical intersections with the ground state i s monitored. We find a bifurcation of the wave packet already in the excite d state so that both conical intersections are reached but by different par ts of the wave packet. By manipulating the initial conditions the branching ratio can be influenced. (C) 2000 American Institute of Physics. [S0021-96 06(00)00511-0].