The rotational excitation of methanol by para-hydrogen

Abstract

In this paper, we present the results of calculations of cross-sections and rate coefficients for rotationally inelastic transitions in methanol (CH 3OH), in its ground torsional state, induced by collisions with para-H2 in its ground rotational state. The interaction potential was calculated by means of many-body perturbation theory, and the corresponding cross-sections computed using the coupled states approximation to the Schrödinger equation for the CH3OH-H2 system. The rate coefficients have been calculated for kinetic temperatures in the range 5 ≤ T ≤ 200 K. We found that, although the rate coefficients for para-H 2 are qualitatively similar to those computed for He, the collisional propensity rules are less pronounced when the perturber is para-H2 than when it is He. The rate coefficients for CH3OH-H2 tend to be larger than for CH3OH-He, a tendency which is anticipated from a comparison of the corresponding interaction potentials. Although our discussion is in terms of E-type methanol, the analogous results for A-type methanol and para-H2 are also available. Our calculations are the first to consider H2 as a collision partner for CH3 OH and are directly relevant to the interpretation of observations of methanol in the interstellar medium.