Spectroscopic, collisional, and thermodynamic properties of the He–CO2 complex from an <i>ab initio</i> 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 εm=−50.38 cm−1 at Rm=5.81 bohr for the “T”-shaped geometry of the complex, and a local one of εm=−28.94 cm−1 at Rm=8.03 bohr for the linear He⋅⋅⋅O=C=O 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 ν3 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 ν4 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 ν5 bending mode of the complex have been used to compute the transition frequencies in the ν5 hot band of He–CO2. A tentative assignment of the transitions observed in the ν5 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 ν3 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.