Close-coupling and coupled state calculations of argon scattering from normal methane

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Abstract
Close-coupling and coupled states integral, differential, and total cross sections in an argon–methane scattering system are presented for all the irreducible representations of the tetrahedral symmetry. From symmetry considerations the rotational wave functions, which transform according to each of the irreducible representations, are defined. A new interaction potential for the Ar–CH4 system is used. The spherically symmetric part of the potential is due to U. Buck, and was found by fitting a flexible Morse-spline–Morse-spline–van der Waals curve to total differential cross section. In the present paper an anisotropic term was added to the potential. Scattering from normal methane consisting of a mixture of all of the symmetries is discussed. Close-coupling and coupled states cross sections are compared for all the representations. Errors in the coupled states approximation run to 30%. Calculations with a Lennard-Jones interaction potential show that the magnitude of the percentage error in the coupled states cross sections is potential dependent. Coupled states integral, differential, and total cross sections for normal methane are presented over the energy range from 246 to 582 cm−1 (30–72 meV). The effect of compound state resonances on this system is illustrated.