Ab initio study of nonadiabatic interactions in the photodissociation of ketene

Ab initio calculations have been carried out on potential-energy surfaces for the photodissociation of ketene. S-0 and S-1 state cross extensively around the Franck-Condon (F-C) region upon C-C-O bending, and the S-1-->S-0 internal conversion is expected to be very efficient. S-1 and T-1 stay close in energy in the F-C region, but do not couple strongly due to the small spin-orbit coupling, and direct S-1-->T-1 intersystem crossing is unlikely. The triplet state: which produces the ground-state products is likely to be formed via the process S-1-->S-0-->T-n.S-0 crosses with the lowest triplet state (T-1 or T-2) at rather low energy near the triplet minimum. The S-0/T-n 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. (C) 1997 American Institute of Physics.