Collisional excitation of lambda doublet transitions in CH and OH

Excitation of the lowest ..lambda.. doublet levels of CH and OH by collisions with H, H/sub 2/, and He and subsequent radiative decay is calculated on the basis of the H/sub 2/CO rotational doublet levels. Molecular asymmetry plays an important role. It is shown that under a wide range of temperatures and densities characteristic of interstellar clouds, population of the ..lambda.. doublet of CH will very likely be inverted by collisions with H, H/sub 2/, or He. This appears to account for the emission and maser amplification frequency found for this transition of CH. The approximations used give less certain results for excitation of the ..lambda.. doublet in OH, but indicate that collisions with H may produce a population inversion, while those with H/sub 2/ or He have the opposite effect, giving an excess population in the lower of two ..lambda.. doublet levels. (AIP)