The torsional excitation of methanol by helium

Abstract

We report the results of coupled channels calculations of cross-sectionsfor torsionally elastic and inelastic transitions in E-type methanol(CH3OH), with helium as the colliding partner. The dependence of theCH3OH–He interaction potential on the internal rotation (torsional) angle wasdetermined using second-order many-body perturbation theory. Themethanol basis comprised levels belonging to the ground torsional state(ν = 0) and the first excitedtorsional state (ν = 1). The collisional ‘propensity rules’ observed in the case of torsionally elasticcollisions were found not to apply to torsionally inelastic transitions between states ofν = 0 and 1. We assessed the effect of the torsional coupling on the torsionally elastic cross-sectionsand found changes of no more than about 30% at the highest collision energy considered(500 cm−1). The cross-sections for torsionally inelastic transitions were found to be typically twoorders of magnitude smaller than for torsionally elastic transitions.