QUANTUM CALCULATIONS OF TRANSPORT AND RELAXATION PROPERTIES FOR THE HELIUM-METHANE SYSTEM

Quantum calculations are reported for He atoms in a gaseous mixture in volving the nonlinear polyatomic system CH4. The helium-methane intera ction was taken from earlier crossed beam studies [11] and the cross-s ections were computed with two different coupling schemes for the angu lar momenta of the tensorial basis: close coupling at low energies and coupled states up to 3500 cm(-1). Results for various coefficients ar e compared and extrapolation methods are discussed for extending them to higher temperatures. A final comparison between measured diffusion coefficients, measured mixture viscosities and the present calculation s shows unequivocally that fairly realistic results can be obtained fo r the calculated transport properties of larger polyatomics. To the au thor's knowledge, it is the first time that such a comparison has been discussed using rigorous quantum methods for a mixture containing a n onlinear molecular partner.