Size effects in the structure and dynamics of CO2 clusters

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Abstract
Clusters are formed in a free jet expansion and studied by electron diffraction, in the range 102–105 CO2 molecules per cluster. When clusters contain more than about 103 molecules, crystallographic methods provide mean structural and dynamical parameters while a comparison between diffraction patterns and calculated diffraction functions of a simple homogeneous model is used in the case of smaller clusters. CO2 clusters are found to present the bulk cubic structure even when they contain less than some 102 molecules. When decreasing the mean cluster size down to some 103 molecules, we observe a linear decrease in the lattice parameter and an increase in the rms atomic displacements, this increase being larger for O atoms than for C atoms due to the molecular librational motions. The calculated diffraction functions make it possible to study quantitatively the correlated motions of nearest neighbor molecules and are in very good agreement with the experimental curves in the range 102–103 molecules per cluster.