title: Ab Initio Trajectory Study on Triplet Ketene Photodissociation via Statistical Sampling of the Crossing Seam.
abstract: Triplet ketene exhibits a steplike structure in the experimentally observed dissociation rate, but its mechanism is still unclear despite many theoretical efforts. A previous surface-hopping simulation at the CASSCF level suggests that nonadiabatic transition from the S0 to T1 states creates the T1 species in a highly nonstatistical manner, which raises the question of whether the use of statistical rate theory is valid in itself for the T1 state. Here, we study this problem by performing ab initio trajectory simulation at the multireference second-order Möller-Plesset perturbation (MRMP) level of theory. Since the MRMP theory is too expensive for such a trajectory calculation, we first construct dual-level potential energy surfaces (PESs) for the S0 and T1 states by calibrating the PESs at the B3LYP level with a limited set of MRMP energies. We then introduce the assumption of vibrational equilibrium on the S0 surface and characterize the S0 → T1 crossing points using the conditional microcanonical distribution on the S0/T1 seam surface. The latter distribution is obtained by running a constrained trajectory on the seam surface by use of an efficient SHAKE-like method. Subsequently, we propagate a number of T1 trajectories from the seam surface to obtain the dissociation rate. The result shows that (i) the S0 → T1 crossing points are localized mainly in the T1 reactant region; (ii) the lifetime on the T1 surface is about 30 ps at the MRMP level, which is 2 orders of magnitude greater than the previous estimate obtained from the surface-hopping simulation at the CASSCF level (∼100 fs); and (iii) the calculated T1 dissociation rate agrees reasonably well with classical transition state theory. These results suggest that the T1 dissociation is rather statistical, given that the T1 trajectories are initiated from the conditional microcanonical distribution on the seam surface.
authors.affiliation: Department of Chemistry, Kyoto University , Kyoto 606-8502, Japan.
authors.forename: Yusuke
authors.initials: Y
authors.lastname: Ogihara
authors.affiliation: Department of Chemistry, Kyoto University , Kyoto 606-8502, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Yamamoto
authors.affiliation: Department of Chemistry, Kyoto University , Kyoto 606-8502, Japan.
authors.forename: Shigeki
authors.initials: S
authors.lastname: Kato