Size effects in the structure and dynamics of CO2 clusters

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.