Multistate coupled quantum dynamics of photoexcited cytosine in gas-phase: Nonadiabatic absorption spectrum and ultrafast internal conversions

Quantum dynamics simulations, with the Multiconfigurational Time Dependent Hartree method, are used to assign the very broad absorption spectrum of Cytosine in gas phase and study the relation between spectral features and the ultrafast internal conversions among its excited states. For each of the four populated tautomers of Cytosine we built a Linear Vibronic Coupling model, comprising all the low energy excited states up to ~6.5 eV (7-9 states). We used Density Functional Theory and a general diabatization scheme based on the projection of the excited states on a set of reference ones. Vibronic progressions and inter-state couplings dominate the spectral shape, which is in nice agreement with experiment. Inter-state couplings contribute to the loss of vibronic resolution and to the spread of the absorption intensity along a large energy range. Their importance is different for each tautomer and is connected to the different decay times of the bright states.