Photodissociation processes in the OH molecule

Abstract

A systematic investigation of the excited states that may participate in the photodissociation of the OH molecule has been carried out by ab initio calculations which use two different self-consistent field with configuration-interaction methods. Potential energy curves for states of 2Σ+, 2Σ−, 2Π, and 2Δ symmetries have been computed and the transition dipole moments connecting the states have been obtained. Photodissociation cross sections for absorption from the v″=0 vibrational level of the ground X 2Π state into the repulsive 1 2Δ, B 2Σ+, and 2 2Π states are reported. It is shown that the 1 2Δ and B 2Σ+ states, together with the 1 2Σ− state considered in an earlier study, are important channels for photodissociation. Photodissociation may take place also following absorption into bound electronic states and absorption oscillator strengths for these transitions have been determined. The mechanisms by which the bound states can be dissociated are discussed and estimates of the dissociation efficiencies of the mechanisms are made. Absorption into the bound 3 2Π state may be an important dissociation channel. Calculations of the interstellar photodissociation rate have assumed that photodissociation proceeds by absorption into the C 2Σ+ state for which a large oscillator strength had been adopted. We find that the C 2Σ+ channel is not significant in photodissociation and we present new estimates of the interstellar photodissociation rates. Our calculations demonstrate that OH is dissociated by the absorption of Lyman alpha radiation, a channel of particular significance in shocked interstellar gas and in cometary atmospheres.