Electron diffraction studies of the kinetics of phase changes in molecular clusters.

Abstract

A method for monitoring the time evolution of freezing and of solid-state transformations is developed. Large molecular clusters, formed in a supersonic nozzle expansion, are probed in flight outside the nozzle by an electron beam. The measured rate of transition corresponds to the rate of nucleation. Nucleation must be homogeneous, because clusters in the supersonic jet are isolated from catalysts and from each other. Because clusters are small ($\sim$100 A) they cannot transform during the $\sim$200 $\mu$s timescale of the experiment unless they attain a nucleation rate about fifteen orders of magnitude higher than those typically found in studies of nucleation. Clusters of CCl$\sb4$ and clusters of CH$\sb3$CCl$\sb3$ were observed to freeze at fractional undercoolings of about 0.3 to the crystalline phase (Ib) stable at the freezing point, rather than to the metastable fcc solid often encountered. Experimental nucleation rates were analyzed in terms of the classical theory of homogeneous nucleation to determine the interfacial free energies between the solid and the melt for both compounds. Values of the free energies of the solid-melt interfaces agree well with those predicted for non-metals by Turnbull's empirical relation. Rates of transitions between two crystalline phases were determined in the systems SeF$\sb6$ and (CH$\sb3)\sb3$CCl. In neither case do molecular translational jumps occur in the undercooled parent phase on the timescale of the experiment. Reorientational jumps, however, take place rapidly and their frequency is taken to correspond to the frequency of crossing the interface between the matrix and the nucleus postulated in nucleation theory. Selenium hexafluoride is known to transform by means of molecular reorientation. Calculations based on this treatment suggest that SeF$\sb6$, cooled below the temperature at which clusters transform in experiment, could attain nucleation rates five to seven orders of magnitude higher than the measured rate, a result consistent with observations of the same transformation in molecular dynamics studies. Interfacial free energies derived for the boundaries between the crystalline phases appear to follow a relation analogous to Turnbull's for solid-melt interfaces.