| Résumé | Infrared spectra of three isotopic forms of the weakly bound CO₂–He van der Waals complex have been studied in the region of the CO₂ v₃ fundamental band around 2300 cm⁻¹, using a tunable diode laser to probe a pulsed supersonic expansion. The complex is a T-shaped near-oblate asymmetric rotor, and it is found that ¹⁸O isotopic substitution is sufficient to interchange the a and b inertial axes. For the symmetric isotopes, such as the normal species and the ¹⁶O¹³C¹⁶O and ¹⁸O¹³C¹⁸O forms studied here, half of the normal rotational levels of the complex are missing due to the effects of ¹⁶O (or ¹⁸O) interchange symmetry. However, for asymmetrically substituted ones, such as ¹⁶O¹³C¹⁸O, all rotational levels are present. Moreover, for the asymmetric isotope, both a- and b-type transitions were observed, so that the spectrum was much richer. The CO₂–He system is of interest both as a benchmark for intermolecular potential energy surface calculations, and because CO₂ is a valuable probe molecule for helium cluster spectroscopy. |
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