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Infrared spectra of (CO$_2$)$_2$-Rg trimers, Rg = Ne, Ar, Kr, and Xe

High resolution spectra of (CO$_2$)$_2$-Rg trimers (Rg = Ne, Ar, Kr, and Xe) in the region of the CO2 nu3 fundamental (~2350 cm$^{-1}$) are reported, using a tunable OPO laser source to probe a pulsed supersonic slit jet expansion. These (CO$_2$)$_2$-Rg transitions tend to be hidden among stronger spectra due to other species, such as CO$_2$-Rg and (CO$_2$)$_2$. Each trimer consists of a (CO$_2$)$_2$ unit which is similar to the free carbon dioxide dimer (planar parallel staggered) plus an Rg atom located out-of-plane on the dimer C$_2$ symmetry axis, but the (CO$_2$)$_2$ unit may not remain exactly planar in the dimer. Experimental structures show that the C-Rg bond lengths in the trimers are similar to those in the corresponding CO$_2$-Rg dimers. As well, the vibrational band origin shifts, relative to (CO$_2$)$_2$ itself, are similar to those of CO$_2$-Rg relative to CO$_2$.