  Ab initio molecular dynamics including non-adiabatic and spin-orbit couplings
on equal footing is used to unravel the deactivation of cytosine after UV light
absorption. Intersystem crossing (ISC) is found to compete directly with
internal conversion in tens of femtoseconds, thus making cytosine the organic
compound with the fastest triplet population calculated so far. It is found
that close degeneracy between singlet and triplet states can more than
compensate for very small spin-orbit couplings, leading to efficient ISC. The
femtosecond nature of the intersystem crossing process highlights its
importance in photochemistry and challenges the conventional view that large
singlet-triplet couplings are required for an efficient population flow into
triplet states. These findings are important to understand DNA photostability
and the photochemistry and dynamics of organic molecules in general.
