Infrared Spectroscopy and Mass Spectrometry of CO2 Clusters during Nucleation and Growth.

CO2 clusters with 2 to 4300 molecules are characterized with mass spectrometry and infrared spectroscopy in the uniform postnozzle flow of Laval expansions at constant temperatures of ∼29 and ∼43 K. The mass spectra provide independent, accurate information on the cluster size distributions and through magic numbers also on cluster structures. The experimental results are complemented with force field, quantum chemical, and vibrational exciton calculations. We find our data to be consistent with predominantly fcc cuboctahedral structures for clusters with more than about 50 molecules. Infrared spectra of cluster size distributions with average sizes above 140-220 molecules are completely dominated by the features from the larger cuboctahedral clusters in the distribution. For very small clusters, exciton simulations predict a pronounced broadening of the infrared band as soon as the average cluster size exceeds about five molecules. The nucleation behavior of CO2 under the present conditions is found to be barrierless in agreement with similar trends previously observed for other compounds at very high supersaturation.