Predominance of Nonequilibrium Dynamics in the Photodissociation of Ketene in the Triplet State

Abstract

The photodissociation of ketene is studied using direct surface-hopping classical trajectories where the energy and gradient are computed on the fly by means of state-averaged complete active space self-consistent field with a double-ζ polarized basis set. Three low-lying electronic states, singlets S0 and S1 and triplet T1, are involved in the process of photodissociation of triplet state ketene. We propagated a trajectory, starting at the Franck−Condon geometry on S1, and branched it out into many child trajectories every time the propagating potential energy surface (PES) crossed with another PES. The major photodissociation pathway to the triplet products was found to be S1 → S0 → T1 → CH2(X3B1) + CO(X1Σ+). It has been found that (1) the S0−T1 nonadiabatic transition creates the T1 species nonstatistically at restricted regions of phase space and (2) a large fraction of the T1 species thus created dissociates almost immediately, leaving no time for equilibration of internal degrees of freedom. Whether a specific T1 trajectory dissociates fast or not is determined by the amount of C−C stretch vibration at the S0−T1 branch point. In essence, the above observations suggest strongly that the T1 photodissociation process is highly nonstatistical, thus making equilibrium-based statistical theories inapplicable for computing the dissociation rate.