Ab initio calculations have been carried out on potential-energy surfaces for the photodissociation of ketene. S0 and S1 state cross extensively around the Franck–Condon (F–C) region upon C–C–O bending, and the S1→S0 internal conversion is expected to be very efficient. S1 and T1 stay close in energy in the F–C region, but do not couple strongly due to the small spin–orbit coupling, and direct S1→T1 intersystem crossing is unlikely. The triplet state, which produces the ground-state products is likely to be formed via the process S1→S0→Tn.S0 crosses with the lowest triplet state (T1 or T2) at rather low energy near the triplet minimum. The S0/Tn crossing persists all along the C–C dissociation pathway. As C–C is stretched, the energy of the crossing increases and the crossing structure deviates substantially from the reaction path. These results suggest that, if intersystem crossing at higher potential energy is favored, the rate of reaction may reflect the dynamics of intersystem crossing and that on the triplet surface.

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