Spectra of CO 2 -Rg 2 and CO 2 -Rg-He trimers (Rg = Ne, Ar, Kr, and Xe): Intermolecular CO 2 rock, vibrational shifts and three-body effects.

Weakly bound CO ₂ -Rg ₂ trimers are studied by high-resolution (0.002 cm ^-1 ) infrared spectroscopy in the region of the CO ₂ ν ₃ fundamental band (≈2350 cm ^-1 ), using a tunable optical parametric oscillator to probe a pulsed supersonic slit jet expansion with an effective rotational temperature of about 2 K. CO ₂ -Ar ₂ spectra have been reported previously, but they are extended here to include Rg = Ne, Kr, and Xe as well as new combination and hot bands. For Kr and Xe, a unified scaled parameter scheme is used to account for the many possible isotopic species. Vibrational shifts of CO ₂ -Rg ₂ trimers are compared to those of CO ₂ -Rg dimers, and in all cases the trimer shifts are slightly more positive (blue-shifted) than expected on the basis of linear extrapolation from the dimer. Combination bands directly measure an intermolecular vibrational mode (the CO ₂ rock) and give values of about 32.2, 33.8, and 34.7 cm ^-1 for CO ₂ -Ar ₂ , -Kr ₂ , and -Xe ₂ . Structural parameters derived for CO ₂ -Rg ₂ trimers are compared with those of CO ₂ -Rg and Rg ₂ dimers. Spectra of the mixed trimers CO ₂ -Rg-He are also reported.