A Time-Dependent Picture of the Ultrafast Deactivation of keto-Cytosine Including Three-State Conical Intersections

Using mixed quantum-classical dynamics, the lowest part of the UV absorption spectrum and the first deactivation steps of keto-cytosine have been investigated. The spectrum shows several strong peaks, which mainly come from the S-1 and S-2 states, with minor contributions from the S-3. The semiclassical trajectories, launched from these three states, clearly indicate that at least four states are involved in the relaxation of keto-cytosine to the ground state. Non-adiabatic transfer between the pi pi* and n pi* excited states and deactivation via three-state conical intersections is observed in the very early stage of the dynamics. In less than 100 fs, a large amount of population is deactivated to the ground state via several mechanisms; some population remains trapped in the S-2 state. The latter two events can be connected to the fs and ps transients observed experimentally.