Rotationally and torsionally inelastic scattering of methanol on helium

Abstract

Cross-sections and rate coefficients have been computed for rotationally and torsionally inelastic transitions between levels of the torsional manifolds ν= 0, 1 and 2 of methanol; the collision partner was the helium atom. We find the torsional inelasticity to be most pronounced for transitions between the excited states, ν= 1 and 2. The torsionally inelastic cross-sections are smaller, on average, than the cross-sections for pure rotationally inelastic transitions, by typically an order of magnitude. We compared cross-sections calculated using the full CH3OH-He interaction potential, which is a function of the torsion angle, with the cross-sections computed for the staggered conformation, i.e. for a fixed value of the torsion angle. The agreement between these two sets of results improves with increasing magnitude of the cross-section. Accordingly, the use of previous results, for the rotational excitation of methanol by He and by H2, obtained for the staggered conformation, should not compromise the analysis of observations of methanol in the interstellar medium. © 2010 The Authors Monthly Notices of the Royal Astronomical Society © 2010 RAS.