Infrared spectra of isotopic CO2-He complexes.

Infrared spectra of three isotopic forms of the weakly bound CO(2)-He van der Waals complex have been studied in the region of the CO(2) nu(3) 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 (18)O isotopic substitution is sufficient to interchange the a and b inertial axes. For the symmetric isotopes, such as the normal species and the (16)O(13)C(16)O and (18)O(13)C(18)O forms studied here, half of the normal rotational levels of the complex are missing due to the effects of (16)O (or (18)O) interchange symmetry. However, for asymmetrically substituted ones, such as (16)O(13)C(18)O, 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 CO(2)-He system is of interest both as a benchmark for intermolecular potential energy surface calculations, and because CO(2) is a valuable probe molecule for helium cluster spectroscopy.