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

Symmetry-adapted perturbation theory has been applied to compute the interm olecular potential energy surface of the He-CO2 complex. The ab initio pote ntial has a global minimum of epsilon (m)=-50.38 cm(-1) at R-m=5.81 bohr fo r the "T"-shaped geometry of the complex, and a local one of epsilon (m)=-2 8.94 cm(-1) at R-m=8.03 bohr for the linear He . . .O=C=O structure. The co mputed potential energy surface has been analytically fitted and used in co nverged variational calculations to generate bound rovibrational states of the He-CO2 complex and the infrared spectrum corresponding to the simultane ous excitation of the nu (3) vibration and internal rotation in the CO2 sub unit within the complex. The complex was shown to be a semirigid asymmetric top and the rovibrational energy levels could be classified with the asymm etric top quantum numbers. The computed frequencies of the infrared transit ions in the nu (4) band of the spectrum are in very good agreement with the high resolution experimental data of Weida [J. Chem. Phys. 101, 8351 (1994 )]. The energy levels corresponding to the nu (5) bending mode of the compl ex have been used to compute the transition frequencies in the nu (5) hot b and of He-CO2. A tentative assignment of the transitions observed in the nu (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 broa dening coefficients of the R branch rotational lines of the nu (3) spectrum of CO2 in a helium bath at various temperatures. Very good agreement is fo und with the wealth of experimental results for various rotational states o f CO2 at different temperatures. Finally, we also tested the potential by c omputing 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 comple x. (C) 2001 American Institute of Physics.