Large CO2 clusters studied by infrared spectroscopy and light scattering

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Abstract
Large CO2 clusters were formed by introducing room temperature gaseous mixtures of CO2 in argon into a cryogenic cell at 77 K. Rapid cooling of each mixture resulted in a highly supersaturated CO2 concentration, giving rise to homogeneous nucleation and thus cluster formation [F. F Abraham, Homogeneous Nucleation Theory, Advances in Theoretical Chemistry, Supplement 1 (Academic, New York, 1974), and references therein]. Experimental results will be presented here for CO2 in argon dilutions of 1:104, 1:2×105, and 1:106. Light scattering and infrared absorption techniques have been combined to estimate an average cluster radius of 0.20 μm for the 1:104 dilution sample, and an upper limit in cluster radius of 0.10 μm for the 1:2×105 dilution sample. Therefore, the higher dilution CO2:Ar mixtures led to the formation of smaller cluster sizes. Infrared structure in the ν3‐asymmetric stretching region of the clusters will be discussed. The quantum mechanical exciton model and the classical Mie model are only partially successful in explaining these experimental observations. Weak absorption features have been assigned to the naturally abundant 13C16O2 and 12C16O18O isotopes. The infrared structure attributed to these minority isotopes is relatively invariant with cluster size formed, and can be explained by the exciton model. This analysis suggests that, whatever overall shape the clusters have assumed, the CO2 molecules within the clusters have separations and orientations like those in the bulk crystal. Infrared spectra were collected at regular intervals over a period of 4 h for each sample. A monotonic decrease in the integrated infrared absorbance of the clusters with time was observed, with a characteristic half‐life of 65, 180, and 230 min for the 1:104, 1:2×105, and 1:106 dilution samples, respectively. The average cluster radii obtained from light scattering and infrared absorption measurements have been used to calculate a sedimentation half‐life of 60 and 180 min for the 1:104 and 1:2×105 dilution samples, respectively, which agreed well with the observed disappearance times. A cluster radius of 0.09 μm for the 1:106 dilution experiment is consistent with its sedimentation half‐life of 230 min.