INFINITE ORDER-SUDDEN CROSS-SECTIONS FOR EXCITATION OF OVERALL AND INTERNAL-ROTATION IN CH3OH-HE COLLISIONS

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 DELTAK = 0 transitions, followed by DELTAK = +/- 3. However, torsional excitation is significant only for K-changing collisions, particularly DELTAK = +/- 3. These latter cross sections approach zero as rho (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 rho. These propensities are weaker in the actual barrier cross sections. The results also show a propensity for A +/- <-- --> A +/- over A +/- <-- --> A -/+ in DELTAK = 0 transitions.