Infinite order-sudden cross sections for excitation of overall and internal rotation in CH3OH–He collisions

Abstract

Inelastic scattering cross sections are reported within the infinite-order sudden (IOS) approximation for excitation of overall and internal rotation in CH3OH by collisions with He at an average energy of 500 cm−1, using a self-consistent field (SCF) plus damped dispersion interaction potential. The sudden approximation was used for the overall as well as the internal rotation, which was described using the zero-order internal axis method (IAM). The torsionally elastic cross sections are dominated by ΔK=0 transitions, followed by ΔK=±3. However, torsional excitation is significant only for K-changing collisions, particularly ΔK=±3. These latter cross sections approach zero as ρ (the ratio of the internal rotor moment of inertia to that of the entire molecule about its symmetry axis), approaches unity. Cross sections were also calculated for the high-barrier and zero-barrier limits to internal rotation. The high-barrier limit values are close to those for the actual barrier of 373 cm−1. The zero-barrier cross sections show strong propensities toward particular K→K′, which are quite sensitive to the value of ρ. These propensities are weaker in the actual barrier cross sections. The results also show a propensity for A±↔A± over A±↔A∓ in ΔK=0 transitions.