Determination of Λ-doublet resolved cross-sections for inelastic scattering of OH by para- and normal-H2

Abstract

In this paper we report the measurement of Λ-doublet resolved state-to-state cross sections for inelastic collisions of OH by H2 at a translational energy of 595 cm−1. Experimental values are obtained for transitions from both the upper (f) and the lower (e) Λ-doublet substates of the lowest rotational state (J=3/2) of OH(2Π3/2) to almost all other states within the range of the collision energy. Cross sections for scattering by both para- and normal-H2 have been determined. The main difference between para- and normal-H2 scattering is seen in the Λ-doublet cross section. The results are compared to He-scattering which shows that H2 (J=0) scattering behaves similar to He scattering. When averaged over the Λ-doublet states, the cross sections are in good agreement with the measurements of Andresen et al. [J. Chem. Phys. 81, 571 (1984)], although the conclusions with regard to collisional pumping of interstellar OH masers are different. The quantum calculations of Offer et al. [J. Chem. Phys. 100, 362 (1994)] show a surprisingly good correspondence with the measured values. In this comparison the full initial state distribution of the OH and H2 beams has been taken into account.