Abstract
The authors present the results of calculations of cross sections and rate coefficients for transitions induced by collisions with H2 within the ground-state Lambda doublet of the OH radical. The calculations are relevant to the 18 cm interstellar OH maser emission. The structure observed in the cross section at low energies is attributed to shape resonances. It is found that the combined effects of the rise in the cross section towards threshold and the contribution of the resonances sustain large values of the rate coefficients at low temperature. The authors suggest that the mechanism of collisional inversion of the populations of the ground state Lambda doublet (by rotational excitation followed by radiative decay) will be effectively quenched by collisions within the doublet at low temperatures.
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