An ab initio direct-trajectory study of the photodissociation of ClOOCl

The photodissociation of chlorine peroxide, ClOOCl, is studied with classic al trajectories where the energy and gradient are computed on the fly by me ans of the state-averaged (sa) complete active space self-consistent field (CASSCF) with the DZP(+) basis set. We show that six electronically excited states are involved in the process of decomposition, which proceeds via se veral competing pathways and at least three electronically unique fragment channels. The problem is treated in four-dimensional (4D) (C-2 constraint) and five-dimensional (5D) (planar constraint) frameworks in order to model the mechanisms of synchronous and asynchronous or stepwise dissociation, re spectively. A single trajectory with the initial conditions of a nonvibrati ng, nonrotating molecule is propagated on each excited state surface for an average time of 10 fs for the purposes of determining the early stages of bond breaking. We show that even in such a short propagation time the pathw ay competition can be more or less unambiguously understood. The results in dicate that in the regime of a 308 nm photolysis, the major dissociation fr agments are Cl atoms and O-2 molecules, both in the ground state. The highe r energy regime of a 248 nm photoexcitation yields additional fragments, e. g., ClO(X (2)Pi), O(P-3) and ClOO(X (2)A",1 (2)A'). We have achieved an ove rall qualitative agreement with experiment that more than 70% of the availa ble energy is transferred into the translational energy of the products for the case of the synchronous concerted dissociation. In all the cases, the rotational excitation of produced molecular oxygen is very high, while its vibration is in v=0. Implications of the results on the stratospheric ozone depletion cycle are also presented. (C) 2000 American Institute of Physics . [S0021-9606(00)30238-0].