(SF6)(N) CLUSTERS, 100-LESS-THAN-OR-SIMILAR-TO-N-LESS-THAN-OR-SIMILAR-TO-3000, PRODUCED IN A SF6+NE GAS EXPANSION - SIZE, TEMPERATURE, AND SOLID PHASE TRANSITION

摘要

In this paper, the phase behavior of SF6 clusters is examined experimentally and is discussed in the context of the previous work. SF6 clusters made of 100 to 3000 molecules are produced in a free jet expansion of a Ne+SF6 mixture. Cluster structures are identified by means of electron diffraction methods and ascertained by molecular dynamics (MD) simulations. On warming up the clusters, diffraction patterns display the transition from the monoclinic (low temperature) to the body centered cubic (high temperature) bulk structure, finite size effects appearing in the form of intermediate patterns that correspond to neither structure. MD simulations have shown that these intermediate patterns are due to a progressive rearrangement of the cluster surface prior to the cluster core transition, a process which leads to the observed temperature spread of the transformation. Taking advantage of the sensitivity of diffraction patterns to cluster temperature, SF6 clusters are used to probe the free jet expansion, particularly the cooling efficiency of the carrier gas and the warming effect caused by the crossing of the frontal shock wave. It is found that upon increasing the SF6 mole fraction, clusters become larger and warmer, the high-temperature structure being achieved when the expanding mixture is nearly saturated in SF6, which corresponds to a maximum cluster size. When cold clusters are allowed to cross the frontal shock wave, they warm up and acquire the cubic structure, without any appreciable evaporation. Using line height measurements in the cubic patterns, it is shown that the variation of the Debye-Waller factor, in a large range of sizes, is mainly due to a size effect. Finally, the temperature at which the transition to the cubic structure occurs is found to be constant for clusters made of more than about 1300 molecules; however, it decreases when the clusters get smaller. This result has been confirmed by recent molecular dynamics simulations. (C) 1995 American Institute of Physics. [References: 31]