Doped rare gas clusters up to completion of first solvation shell, CO2–(Rg)n, n = 3–17, Rg = Ar, Kr, Xe

Spectra of rare gas atom clusters containing a single carbon dioxide molecule are observed using a tunable mid-infrared (4.3 µm) 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, and CO2–Krn and CO2–Xen with n = 3, 4, and 5. Each spectrum has (at least) a partially resolved rotational structure, and each yields precise values for the shift of the CO2 vibrational frequency (ν3) 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 and 13) are probably also present in the observed spectra but with band structures that are not well-resolved and, thus, are not recognizable. The spectra of CO2–Ar9, CO2–Ar15, and CO2–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).