Collisional excitation of lambda doublet transitions in CH and OH

A mechanism is described for pumping the CH ground-state doublet and exciting the lowest lambda-doublet levels via collisions with H, H2, or He and subsequent radiative decay. A semiclassical evaluation is made of the interaction potential between the colliding molecules and of the relevant cross sections. An approximate calculation shows that this mechanism is very likely to invert the population of the CH lambda doublet under a wide variety of physical conditions characteristic of either molecular or atomic-hydrogen clouds and could account for the emission and maser amplification often observed for this CH transition. Similar calculations for the OH radical tentatively indicate that the mechanism is probably effective in inverting the OH lambda doublet only in a cloud which is predominantly atomic hydrogen. It is suggested that collisions between OH and H2 or He may give an excess population to the lower of two lambda-doublet levels.