THEORY OF H+OH (2-PI) COLLISIONS AND INTER-STELLAR OH MASER ACTION

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 B 1A′) 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. © 1979 American Institute of Physics.