In a preceding paper, we have described the polyicosahedral structure of small argon clusters containing less than about 50 atoms which were produced in a free jet expansion. Going on with this study, we describe presently the multilayer icosahedral structure of larger argon clusters containing up to about 750 atoms, a value for which the transition to the fcc crystalline bulk structure is found to occur. Cluster models are used in order to study the third icosahedral layer construction, and the transition from a twin to a regular surface arrangement, which is expected to take place around 75 atoms, is observed on experimental patterns. The good agreement between experimental and calculated diffraction functions leads to an estimate of mean cluster sizes, cluster size distributions, and temperature (32±2 K) of clusters containing several hundreds of atoms.
Clusters are produced in a free jet expansion of water vapor. Keeping constant the nozzle diameter (d=0.4 mm) and temperature (T=430 K), an increase in inlet vapor pressure from 1 to 5 bar produces an increase in mean cluster size from several tens to several thousands of molecules per cluster. An electron diffraction analysis provides information about the cluster structure and dynamics. A direct observation of diffraction patterns shows that the largest clusters exhibit mainly a crystalline structure, namely, diamond cubic which is the metastable phase of bulk solid water, whereas the smallest ones are amorphous. In order to elucidate the local order in the latter, a comparison is made between the experimental curves and the diffraction functions calculated for various noncrystalline models. The best agreement is obtained with a model which presents distorted rings of three to six H2O molecules, constructed by cooling a liquid water droplet through a molecular dynamics calculation. In particular, this means that the regular dodecahedral structure often referred to in mass spectroscopy is not a realistic model for neutral clusters.
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