  Spectra of rare gas atom clusters containing a single carbon dioxide molecule
are observed using a tunable mid-infrared (4.3 micron) source to probe a pulsed
slit jet supersonic expansion. There are relatively few previous detailed
experimental results on such clusters. The assigned clusters include CO2-Arn
with n = 3, 4, 6, 9, 10, 11, 12, 15, and 17, as well as CO2-Krn and -Xen with n
= 3, 4, and 5. Each spectrum has (at least) partially resolved rotational
structure, and each yields precise values for the shift of the CO2 vibrational
frequency (nu3) induced by the nearby rare gas atoms, together with one or more
rotational constants. These results are compared with theoretical predictions.
The more readily assigned CO2-Arn species tend to be those with symmetric
structures, and CO2-Ar17 represents completion of a highly symmetric (D5h)
solvation shell. Those not assigned (e.g. n = 7, 13) are probably also present
in the observed spectra, but with band structures which are not well-resolved
and thus not recognizable. The spectra of CO2-Ar9, -Ar15, and -Ar17 suggest the
presence of sequences involving very low frequency (~2 cm-1) cluster
vibrational modes, an interpretation which should be amenable to theoretical
confirmation (or rejection).
