Rotational excitation of OH by H₂: calculations in intermediate coupling

The authors present the results of calculations of cross sections for the rotational excitation of OH by para-H 2 . The calculations are performed in the angular momentum coupling scheme appropriate to the OH radical, intermediate between Hund's cases a and b. Comparison with previous results, obtained in the case b limit, shows that the principal effect of the spin-orbit interaction is to reduce the asymmetries in the cross sections which lead to preferential excitation of a given component of a Lambda doublet. Nevertheless, the asymmetries remain large, and rotational excitation by H 2 may be expected to play an important role in producing the maser emission from the Lambda doublets in the 2 Pi 3/2 ladder, observed in the interstellar medium.