The spin–orbit effect on potential surfaces of NO2 photodissociation

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Abstract
Potential energy surfaces for photodissociation reaction NO2→NO(2Π)+O(3P) have been studied by ab initio calculations. The effect of spin–orbit interaction on the potential surface features was studied near the product region. All the 12 potential surfaces asymptotically correlated to the NO(2Π)+O(3P) limit were obtained by the state‐averaged complete‐active‐space‐self‐consistent‐field (CASSCF) method. The adiabatic potential surfaces including the spin–orbit interaction were constructed using the full Breit–Pauli Hamiltonian. It was found that the lowest two states are attractive, while all the other states are repulsive. Assuming that NO2 undergoes the photodissociation on the ground state surface, we obtained the bending‐rotation energy levels along the dissociation coordinate, and the transition state for each bending level was determined. The potential barriers for the vibrationally adiabatic energy curves were consistent with the recent experiments. Using a simplified model based on the infinite order sudden approximation (IOSA) and the Franck–Condon approximation, the product fine structure distribution was estimated, which is in good agreement with the experimental results.