Nonadiabatic transitions, spin-orbit coupling, and prereactive complexes in polyatomic reaction dynamics

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The project intends to contribute to the fundamental understanding of elementary chemical reaction processes in polyatomic systems. Transitions between electronic states which are induced by the approach of reactants will be investigated in detail for systems which consist of more than only three or four atoms. Vibronic and spin-orbit coupling will be explicitly included in the quantum dynamics simulations. Prereactive complexes and their importance for the reaction processes will be described in detail. As prototypical examples intensively studied by detailed experiment and theory, reactions of methane with fluorine and chlorine will be investigated.Atoms as fluorine or chlorine show degenerate electronic states which in reactions with molecules such as methane give rise to vibronic and spin-orbit coupling effects in the entrance channel of the reaction. In previous theoretical simulations, the reactions have been studied only on a single adiabatic potential energy surface and non-adiabatic transitions resulting from vibronic and spin-orbit coupling have not been accounted for. In the present project the non-adiabatic dynamics in the entrance channel of the reactions is investigated. The simulations employ coupled diabatic potential energy surfaces which accurately model the vibronic and spin-orbit coupling. Resonances caused by prereactive complexes are investigated by full-dimensional (12D) quantum dynamics calculations using the multi-layer multi-configurational time-dependent Hartree approach and the effect of non-adiabatic transitions on the reaction process will be studied by trajectory surface hopping calculations. Results will be compared with recent transition state spectroscopy and crossed molecular beam experiments.