Ab initio potential-energy surfaces of O2(XΣg−3,aΔg1,bΣg+1)+O2(XΣg−3,aΔg1,bΣg+1): Mechanism of quenching of O2(aΔg1)

Abstract

Ab initio computational studies were carried out in order to explore the possible mechanisms of quenching of O2(aΔg1) by O2(XΣg−3): the self-quenching of O2(aΔg1) and other energy-transfer processes involving two O2 molecules. All eighteen states arising from two O2 molecules in the XΣg−3, aΔg1, and bΣg+1 states are considered. After scans at the state-averaged complete active space self-consistent field method to identify possible regions of crossing between states belonging to different asymptotes, complete active state second-order perturbation theory high-symmetry optimization and low-symmetry scans established that four different minima on the seams of crossing (MSXs), arising between the a+a manifold and the X+b manifold and responsible for self-quenching: O2(aΔg1)+O2(aΔg1)→O2(XΣg−3)+O2(bΣg+1), have coplanar C2h or C2v symmetries and are only 0.45eV barrier relative to the a+a asymptote and energetically easily accessible. The rate constant for this process was estimated based on the Landau-Zener formalism. The MSXs for quenching of O2(aΔg1) by the ground state O2(XΣg−3):O2(aΔg1)+O2(XΣg−3)→O2(XΣg−3)+O2(XΣg−3) require higher energies and the process is not likely to be important.