Tino G. A. Heijmen^a, Tatiana Korona^b, Robert Moszynski^b, Paul E. S. Wormer^a, and Ad van der Avoird^a

^a Institute of Theoretical Chemistry, NSR Center, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands
^b Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland

Ab initio potential-energy surface and rotationally inelastic integral cross sections of the Ar-CH₄ complex

Abstract

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential-energy surface of the Ar-CH₄ complex. The interaction energy, including high-level intramonomer correlation effects, is found to be dominated by the first-order exchange contribution and the dispersion energy. The ab initio potential has four equivalent minima of _m=-144.30 cm^-1 at R_m=7.00 bohr, for structures in which the argon atom approaches the face of the CH₄ tetrahedron. The computed potential-energy surface has been analytically fitted and used in converged close-coupling calculations to generate state-to-state integral cross sections for rotational excitation of CH₄ in collisions with argon. The computed cross sections are generally in good agreement with the experimental data [W. B. Chapman et al., J. Chem. Phys. 105, 3497 (1996)]. Some discrepancies for the smallest cross sections can be explained by the influence of sequential collision channels, with the use of a master equation approach.

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