Monte Carlo study of liquid benzene and characterization of seeded sulfur-hexafluoride cluster beams by electron diffraction and light scattering.

Abstract

Information from electron diffraction studies of molecular clusters generated in nozzle flow has been combined with other evidence to analyze (1) liquid structure and intermolecular interactions and (2) characteristics of seeded cluster beams. For the former application, a variety of benzene interaction models were examined. Neither six site models nor twelve site models devoid of electrostatic interactions were satisfactory. Electrostatic interactions reproducing the observed quadrupole moment were satisfactory only if they were distributed over the entire molecule; a point quadrupole or six point dipoles were inadequate. A twelve site model based on a modification of a quantum computation on benzene dimers by Karlstrom et al proved to be superior to all prior models in accounting for liquid structure and thermodynamics while accurately reproducing the crystal structure and heat of sublimation. It was found that the crucial electrostatic interactions (r$\sp{-1}$) could be modeled over the relevant range of atomic contacts with little loss in accuracy by the computationally more economical form r$\sp{-2}$. Cluster beams of microcrystalline SF$\sb6$ in rare gas carrier were generated in flow through a miniature Laval nozzle and passed through either a conventional conical skimmer or a slit skimmer prior to being probed by an electron beam which scanned the spatial distribution of the transmitted supersonic jet. Electron diffraction monitored the local SF$\sb6$/carrier composition in the jet, the fraction of SF$\sb6$ condensed and (in the case of Ar) carrier condensed into clusters, the cluster temperature, and the distribution of monoclinic(cold)/bcc(warm) SF$\sb6$ phases nucleated. A large mass fractionation was observed that depended strongly on the type of skimmer and expansion conditions. Fractionation was promoted by cluster formation, in part, apparently, by a radial temperature gradient driving the lighter species away from the centerline. The skimmer design markedly influenced the crystalline phases seen for the clusters. Cluster trajectories radiated in nearly straight lines from a point in the nozzle to a cold trap downstream. Cluster diameters inferred from light scattering agreed in magnitude with the crystal domain sizes inferred from electron diffraction.