Dynamics of the photodissociation of triplet ketene

Calculations of the microcanonical dissociation rate for vibrationally excited ketene on the first excited triplet surface (T1) are presented. The calculations utilize the quantum reactive scattering methodology of absorbing boundary conditions (ABC) with a discrete variable representation (DVR) to obtain the cumulative reaction probability for dissociation over the barrier. Model 1- and 2-degree of freedom potential energy surfaces for the T1 surface were obtained by fitting to the best available ab initio structures, energies, and frequencies. The dissociation rates in these reduced-dimensionality calculations give good overall agreement with the experimentally measured rates, although the steplike features seen in the experiments are washed out by the tunneling through the narrow barrier predicted in the ab initio calculations. Further model calculations reveal that a barrier frequency of approximately 50–100i cm−1 is required to recover the step structure seen experimentally, which suggests that there is either another transition state region on the T1 surface farther out towards the product channel, or that there is surface-hopping dynamics taking place between the T1 and S0 ketene potential energy surfaces, or that the ab initio barrier frequency is simply too large.