  The photoinduced excited-state dynamics of the keto and enol forms of
cytosine is investigated using ab initio surface hopping in order to understand
the outcome of molecular beam femtosecond pump-probe photoionization
spectroscopy experiments. Both singlet and triplet states are included in the
dynamics. The results show that triplet states play a significant role in the
relaxation of the keto tautomer, while they are less important in the enol
tautomer. In both forms, the T$_1$ state minimum is found too low in energy to
be detected in standard photoionization spectroscopy experiments and therefore
experimental decay times should arise from a simultaneous relaxation to the
ground state and additional intersystem crossing followed by internal
conversion to the T$_1$ state. In agreement with available experimental
lifetimes, we observe three decay constants of 7 fs, 270 fs and 1900 fs - the
first two coming from the keto tautomer and the longer one from the enol
tautomer. Deactivation of the enol form is due to internal conversion to the
ground state via two S$_1$/S$_0$ conical intersections of ethylenic type.

Chemical Physics