ROTATIONAL-EXCITATION OF OH BY PARA-H2 AND ORTHO-H2

Previous calculations of cross sections for the rotational excitation of OH by H2 have been restricted to collisions with para-H2 in its rotational ground state (J=0). In the present letter, the authors present the first results for ortho-H2 (J=1). They find that the collisional propensity rules, favouring excitation of a given component of a Lambda -doublet, are less pronounced in collisions with ortho- than with para-H 2. Comparison with cross-beam measurements, at a relative energy of 680 cm-1, suggests that discrepancies previously attributed to inadequacies in the potential energy surfaces may, in part, be due to the presence of excited rotational states in the H2 beam.