  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$_2$.
