-doublet population inversion in collisions of OH, OD, CH, CD and NH+

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Abstract
The results of a new approach to the problem of the collisional step in the pumping cycle for OH and CH masers are reported. Rotationally inelastic collisions between OH and CH and both closed- and open-shell collision partners are considered using an expression derived from scattering theory. This is in contrast to previous work of other authors whose predictions have been based on the model of Gwinn and Townes. Relative crosssections for the excitation of pairs of Λ-doublets, derived using simple model interaction potentials, are tabulated. It is shown how Λ-doublet population inversion can arise in OH and CH. An important result is that in some cases H atoms and H2 show opposite behaviour, one partner cooling where the other leads to inversion. We also show how collisions invert the Λ-doublet populations of certain higher rotational levels of OH. The implications for maser action are discussed and reveal excellent qualitative agreement with observation. Λ-doublet population inversion in OD, CD and NH+ is also considered.

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