The photoinduced excited-state dynamics of the keto and enol forms of cytosine have been investigated by using ab initio surface-hopping to gain an understanding of the outcome of molecular beam femtosecond pump-probe photoionisation spectroscopy experiments. Both singlet and triplet states were included in the dynamics. The results show that triplet states play a significant role in the relaxation of the keto tautomer, whereas they are less important in the enol tautomer. In both forms, the T1 state minimum was found to be too low in energy to be detected in standard photoionisation spectroscopy experiments and therefore experimental decay times should arise from simultaneous relaxation to the ground state and additional intersystem crossing followed by internal conversion to the T1 state. In agreement with available experimental lifetimes, we observed three decay constants of 7, 270 and 1900 fs, the first two coming from the keto tautomer and the third from the enol form. Deactivation of the enol tautomer is due to internal conversion to the ground state through two ethylenic-type S1/S0 conical intersections.
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