The UV absorption of ClO -: Part 2.: Predissociation of the A2ΠΩ state studied by ab-initio and Fermi golden rule calculations

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 A(2)Pi(Omega) state (Omega=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 X (2)Pi and A (2)Pi 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 X (2)Pi, A (2)Pi and a (4)Sigma(-) potentials are bound; though the latter has yet to be observed experimentally. All the other states are repulsive, and the lowest ten cross the A (2)Pi(3/2) component at energies below the upsilon' = 6 vibrational level. Using arguments based on electronic configurations, candidate states for the predissociation of the A (2)Pi(Omega) 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 A (2)Pi(3/2) and A (2)Pi(1/2) states. The vibrational dependence of the predissociation rate in the A (2)Pi(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 A (2)Pi(3/2) and A (2)Pi(1/2) components restricts the principal players in the predissociation to three repulsive states: 1 (4)Sigma(Omega)(+), 2 (4)Sigma(Omega)(-) and 3 (2)Pi(Omega). The possibility of coupling between the A(2)Pi(3/2) component and the 1 (2)Delta state is also suspected. A marked similarity is revealed for the variation of predissociation rates with upsilon' for the A (2)Pi(3/2) and A (2)Pi(1/2) components despite a spin-orbit state dependence of the absolute rates for individual upsilon' levels. A repulsive 2 (2)Sigma(+) 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 (2)Pi state correlating diabatically to the Cl(P-2) + O(D-1) dissociation limit.