On the theory of H+OH (2Π) collisions and interstellar OH maser action

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Abstract
The role of H atom collisions as a pump for the 18 cm OH maser lines in interstellar sources is considered. Theory of 2Π molecules collision with a doublet‐state atom is formulated. Explicit expressions for the interaction matrix between the sublevels of the H(2S)+OH(2Π) system are derived, and correlated to the H2O Born–Oppenheimer states. The interaction matrix thus derived is used to obtain cross‐sections and rate constants between (and within) the j=3/2 and J=5/2 rotational manifolds, using the exponential Born method. The H+OH collision is shown to selectively excite the upper, (ε=+1), Λ‐doubled state of the j=5/2 manifold. This selectivity is shown to result from the attractive nature of the X 1A′ component, which is also helpful in bringing about rotational excitation. The repulsive (3A″, 3A′ and B1A′) components are shown to have smaller propensities for rotational excitation and selectivity. The collisional rate constants are used in a simple cloud model, in which radiative decay and effects of background radiation are also incorporated. The model is solved for a wide range of H atom densities. Nonthermal, and often inverted, Λ level populations are shown to exist for a large variety of kinetic temperatures and densities, thus suggesting that collisions with H serve as a major pump source of all four 18 cm emission lines.