Tatiana Korona^a, Robert Moszynski^a, Franck Thibault^b, Jean-Michel Launay^b, Béatrice Bussery-Honvault^b, Joël Boissoles^b, and Paul E. S. Wormer^c

^a Department of Chemistry, University of Warsaw, Pasteura 1, 02-093 Warsaw, Poland
^b Laboratoire PALMS, UMR 6627 du CNRS, Campus de Beaulieu, Université de Rennes I, 35042 Rennes Cedex, France
^c Institute of Theoretical Chemistry, Nijmegen - SON Research Center, University of Nijmegen, Toernooiveld 1, 6525 ED Nijmegen, The Netherlands

Spectroscopic, collisional, and thermodynamic properties of the He-CO₂ complex from an ab initio potential: Theoretical predictions and confrontation with the experimental data

Abstract

Symmetry-adapted perturbation theory has been applied to compute the intermolecular potential energy surface of the He-CO₂ complex. The ab initio potential has a global minimum of _m=-50.38 cm^-1 at R_m=5.81 bohr for the "T"-shaped geometry of the complex, and a local one of _m=-28.94 cm^-1 at R_m=8.03 bohr for the linear He···OCO structure. The computed potential energy surface has been analytically fitted and used in converged variational calculations to generate bound rovibrational states of the He-CO₂ complex and the infrared spectrum corresponding to the simultaneous excitation of the 3 vibration and internal rotation in the CO₂ subunit within the complex. The complex was shown to be a semirigid asymmetric top and the rovibrational energy levels could be classified with the asymmetric top quantum numbers. The computed frequencies of the infrared transitions in the 4 band of the spectrum are in very good agreement with the high resolution experimental data of Weida et al. [J. Chem. Phys. 101, 8351 (1994)]. The energy levels corresponding to the 5 bending mode of the complex have been used to compute the transition frequencies in the 5 hot band of He-CO₂. A tentative assignment of the transitions observed in the 5 band with the quantum numbers of the asymmetric rotor is presented. As a further test of the ab initio potential we also report the pressure broadening coefficients of the R branch rotational lines of the 3 spectrum of CO₂ in a helium bath at various temperatures. Very good agreement is found with the wealth of experimental results for various rotational states of CO₂ at different temperatures. Finally, we also tested the potential by computing the second virial coefficients at various temperatures. Again, the agreement between theory and experiment is satisfactory, showing that the ab initio potential can reproduce various physical properties of the complex.

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