The role of H atom collisions as a pump for the 18 cm OH maser lines in interstellar sources is considered. Theory of2Pgr; molecules collision with a doublethyphen;state atom is formulated. Explicit expressions for the interaction matrix between the sublevels of the H(2S)+OH(2Pgr;) system are derived, and correlated to the H2O Bornndash;Oppenheimer states. The interaction matrix thus derived is used to obtain crosshyphen;sections and rate constants between (and within) thej=3/2 andJ=5/2 rotational manifolds, using the exponential Born method. The H+OH collision is shown to selectively excite the upper, (egr;=+1), Lgr;hyphen;doubled state of thej=5/2 manifold. This selectivity is shown to result from the attractive nature of theXthinsp;1Aprime; component, which is also helpful in bringing about rotational excitation. The repulsive (3APrime;,3Aprime; andB1Aprime;) 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, Lgr; 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.
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