Intersystem crossing in photochemistry

When molecules absorb light they are electronically excited and they can explore different regions of the excited potential energy surfaces (PES).  Particularly interesting are regions where two or more PES cross, since they allow for a efficient deactivation from one to another state. Such, so-called, non-radiative process can be classified as internal conversion or intersystem crossing depending on whether the multiplicity is kept or not in the transition. While the first process is theoretically well studied using both, quantum chemical and dynamical methods, dynamical simulations including intersystem crossing are much less common. One way to simulate the dynamics of light-induced  processes is to employ ab initio molecular dynamics. The latter method has been recently extended in our group to include spin-orbit couplings, so that intersystem crossing dynamics can be studied in full dimensionality. In this presentation I will describe our recent progress in describing spin-forbidden processes, with the help of few showcases. Particularly interesting is that intersystem crossing can be ultrafast in molecules with light atoms, thus taking place in a similar femtosecond time scale as internal conversi