| Abstract
| - A three-state conical intersection between the ground state and the (π,π*) and (nO,π*) singlet excited statesof cytosine is the topological feature that dominates the ultrafast decay of singlet excited cytosine. The three-state intersection is associated with seams of intersection between pairs of states (S1/S0 and S2/S1, respectively),and the resulting topology has been mapped out with CASSCF and CAS-PT2 calculations. The minimum-energy path for the optically active (π,π*) state lies on the S1 surface, and decay to the ground state takesplace at the S1/S0 seam. On the other hand, the region of the S2/S1 seam must be traversed before accessingthe conical intersection with the ground state and recrossing to S2 becomes possible. Another feature associatedwith the three-state degeneracy is vibronic coupling between the (π,π*) and (nO,π*) excited states (proximityeffect), which lowers the barrier to the S1/S0 seam. From a mechanistic point of view, then, the decay is theoutcome of the interaction between the three states. The results also suggest that the experimental excited-state lifetime is the effect of two factors, an energetically accessible region of S1/S0 degeneracy and a regionwhere the decay can be slowed because of recrossing to S2.
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