Photodissociation pathways in the simplest Criegee intermediate: a semi-classical investigation

The dissociation of the simplest Criegee intermediate (H2COO) into formaldehyde (H2CO) and oxygen atom (O) is very important in the atmospheric chemistry. In this study, we investigate the photodissociation of the O-O bond of H2COO by simulating the dynamics of the process on the fitted multi configurational adiabatic potential energy surfaces (PESs). Tully's fewest-switches surface hopping (FSSH) method is used for the simulation. The FSSH trajectories are initiated on the lowest optically-bright singlet excited state (S-2) and propagated along the O-O coordinate. Some of the trajectories end up on energetically lower PESs as a result of radiationless transfer through conical intersections. However, all the trajectories lead to O-O bond dissociation via one of the two channels. The simulation results demonstrate that the restricted O-O motion dissociates H2COO into singlet fragments via the lower energy channel. The coupling of electronic states along O-O may account for this.