Interplay of electronic and nuclear degrees of freedom in a femtosecond-scale photochemical reaction

The intricate dynamical processes in photochemical reactions are not fully accessible to either experiment or conventional theory. Here, we outline a technique for simulations in photochemistry, which employs classical trajectories for the nuclei moving in a mean field, with the electrons coupled to a laser pulse by the time-dependent Peierls substitution. We demonstrate that it provides an illuminating description of photoisomerization. One observes a nontrivial sequence of events which include multiple electronic excitations, conversion of double bonds to single bonds (and vice versa), nonadiabatic depopulation of excited levels at avoided crossings, vibrational energy redistribution, and an elegant interdependence of the various electronic and vibrational degrees of freedom.