Predominance of nonequilibrium dynamics in the photodissociation of ketenein the triplet state

The photodissociation of ketene is studied using direct surface-hopping cla ssical trajectories where the energy and gradient are computed on the fly b y means of state-averaged complete active space self-consistent field with a double-zeta polarized basis set. Three low-lying electronic states, singl ets S-0 and S-1 and triplet T-1, are involved in the process of photodissoc iation of triplet state ketene. We propagated a trajectory, starting at the Franck-Condon geometry on S-1, and branched it out into many child traject ories every time the propagating potential energy surface (PES) crossed wit h another PES. The major photodissociation pathway to the triplet products was found to be S-1 --> S-0 --> T-1 --> CH2((XB1)-B-3) + CO(X(1)Sigma (+)). It has been found that (1) the S-0-T-1 nonadiabatic transition creates the T-1 species nonstatistically at restricted regions of phase space and (2) a large fraction of the T-1 species thus created dissociates almost immedia tely, leaving no time for equilibration of internal degrees of freedom. Whe ther a specific T-1 trajectory dissociates fast or not is determined by the amount of C-C stretch vibration at the S-0-T-1 branch point. In essence, t he above observations suggest strongly that the T-1 photodissociation proce ss is highly nonstatistical, thus making equilibrium-based statistical theo ries inapplicable for computing the dissociation rate.