ELECTRON-DIFFRACTION STUDIES OF THE KINETICS OF PHASE-CHANGES IN MOLECULAR CLUSTERS .2. FREEZING OF CH3CCL3

The time evolution of the freezing of CH3CCl3 has been monitored with the aid of a recently developed technique in which highly supercooled clusters, generated by condensation of vapor in supersonic flow through a Laval nozzle, are probed by electron diffraction. Clusters are observed to freeze into the stable high-temperature solid (Ib) at about 165 K with a nucleation rate of 1.9 x 10(28) m-3 s-1. The time of flight of clusters outside the nozzle before the onset of freezing corresponds to the period of evaporative cooling required to achieve a nucleation rate sufficiently high for freezing to be observed in the supersonic jet, rather than the lag time for homogeneous nucleation, which is much shorter. If results are interpreted in terms of the classical nucleation theory of Turnbull and Fisher, and critical nuclei are assumed to be of phase Ib, the interfacial free energy between the liquid and the solid is found to be 4.7(8) mJ/m2. This result is in satisfactory agreement with estimates derived from Turnbull's empirical relation. The possibility that critical nuclei of the solid might be of the metastable fcc phase (Ia) is discussed, and arguments against this possibility are advanced. Estimates of the growth rate of postcritical nuclei imply that our observations correspond to mononuclear freezing of clusters into single crystals.