  Weakly-bound CO2-Rg2 trimers are studied by high resolution (0.002 cm-1)
infrared spectroscopy in the region of the CO2 nu3 fundamental band (~2350
cm-1), using a tunable OPO to probe a pulsed supersonic slit jet expansion with
an effective rotational temperature of about 2 K. CO2-Ar2 spectra have been
reported previously, but 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 CO2-Rg2 trimers are compared to those of CO2-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 CO2 rock), and
gives values of about 32.2, 33.8, and 34.7 cm-1 for CO2-Ar2, -Kr2, and -Xe2.
Structural parameters derived for CO2-Rg2 trimers are compared with those of
CO2-Rg and Rg2 dimers. Spectra of the mixed trimers CO2-Rg-He are also
reported.
