Molecular dynamics calculations have been used to explore the structure and dynamics of clusters of carbon dioxide, ranging in size up to 164 molecules. The most detailed calculations have been carried out around T=90 K with a view to interpreting the results of infrared data on clusters produced in a seeded argon beam. Under appropriate conditions, in addition to liquid‐like clusters, we have been able to produce clusters with the following structures: (i) an ordered solid—analogous to the bulk crystal, (ii) a bulk–solid core with liquid‐like outer layers, (iii) an amorphous solid, and (iv) an amorphous solid core with liquid‐like outer layers. We have calculated the dispersion of the infrared active intramolecular Q3 mode for these clusters using the transition‐dipole transition‐dipole interaction that is usually invoked to explain the infrared spectrum of the bulk crystal. The resulting spectrum for the Q3 mode of each type of cluster is quite distinct in appearance. This observation suggests that molecular dynamics calculations could be a valuable diagnostic tool in the study of clusters. The dynamical behavior of these clusters has also been investigated, and we find evidence for the existence of large‐amplitude, low‐frequency symmetric breathing modes with frequencies in the range 2–4 cm−1. The overall shape of the experimental Fourier transform infrared spectra of clusters produced in a seeded argon jet is very similar in appearance to the calculated spectrum for a cluster of 116 molecules with an ordered solid core and liquid‐like outer layers.
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Infrared spectra of supersonic jets of carbon dioxide seeded in helium are reported: Features assignable to clusters were observed for the ν2, ν3, ν1+ν3, and 2ν2+ν3 bands. A detailed study was made of the ν3 band, from which it was concluded that the clusters do not adopt the face‐centered cubic crystal structure of solid carbon dioxide. The observed spectra indicate that the cluster geometry is largely determined by pairwise interactions between adjacent molecules. Furthermore, expansions of pure carbon dioxide produced clusters whose spectra indicate motional averaging: such clusters seem to be fluid on a picosecond time scale.
