Quantum dynamical investigation of the simplest Criegee intermediate CH₂OO and its O-O photodissociation channels

The singlet electronic potential energy surfaces for the simplest Criegee intermediate CH₂OO are computed over a two-dimensional reduced subspace of coordinates, and utilized to simulate the photo-initiated dynamics on the S₂ (B) state leading to dissociation on multiple coupled excited electronic states. The adiabatic electronic potentials are evaluated using dynamically weighted state-averaged complete active space self-consistent field theory. Quasi-diabatic states are constructed from the adiabatic states by maximizing the charge separation between the states. The dissociation dynamics are then simulated on the diabatically coupled excited electronic states. The B ← X electronic transition with large oscillator strength was used to initiate dynamics on the S₂ (B) excited singlet state. Diabatic coupling of the B state with other dissociative singlet states results in about 5% of the population evolving to the lowest spin-allowed asymptote, generating H₂CO (X ¹A₁) and O (¹D) fragments. The remaining ∼95% of the population remains on repulsive B state and dissociates to H₂CO (a ³A″) and O (³P) products associated with a higher asymptotic limit. Due to the dissociative nature of the B state, the simulated electronic absorption spectrum is found to be broad and devoid of any vibrational structure.