The UV absorption of ClO Part 2. Predissociation of the A2ΠΩstate studied by ab-initio and Fermi golden rule calculations

Abstract

Ab-initio calculations at the CASSCF and MRCI level have been performed on the valence doublet and quartet electronic states of the ClO radical in order to explore the predissociation dynamics of the A²Π_Ω state (Ω=1/2 or 3/2). The ab-initio calculations neglect spin–orbit coupling. Large basis sets with diffuse orbitals were employed to ensure the calculated X²Π and A²Π potentials match closely the available experimental data. In total, the potential energies of fifteen valence states were computed at a series of internuclear separations in order to map the potential energy curves. The X²Π, A²Π and a⁴Σ^- potentials are bound; though the latter has yet to be observed experimentally. All the other states are repulsive, and the lowest ten cross the A²Π_3/2 component at energies below the v′=6 vibrational level. Using arguments based on electronic configurations, candidate states for the predissociation of the A²Π_Ω state can be selected. Simulations of the predissociation were performed by Fermi golden rule calculations for each vibrational level, using the ab-initio repulsive potentials and RKR curves for the A²Π_3/2 and A²Π_1/2 states. The vibrational dependence of the predissociation rate in the A²Π_3/2 component was used to derive the magnitude of the individual couplings with the repulsive potentials. Comparison between the lifetimes of the vibrational levels in the A²Π_3/2 and A²Π_1/2 components restricts the principal players in the predissociation to three repulsive states: 1⁴Σ_Ω⁺, 2⁴Σ_Ω^- and 3²Π_Ω. The possibility of coupling between the A²Π_3/2 component and the 1²Δ state is also suspected. A marked similarity is revealed for the variation of predissociation rates with v′ for the A²Π_3/2 and A²Π_1/2 components despite a spin–orbit state dependence of the absolute rates for individual v′ levels. A repulsive 2²Σ⁺ state is identified which is likely to be involved in the UV photodissociation of ClO. Similar ab-initio calculations on FO have revealed no bound excited valence ²Π state correlating diabatically to the Cl(²P)+O(¹D) dissociation limit.