SOLUTE FREQUENCY SHIFTS AS A PROBE OF STRUCTURE AND DYNAMICS IN HETEROGENEOUS VAN DER WAAS CLUSTERS

Abstract

Molecular dynamics simulations of $SF_{6}.(Rg){n}$ clusters $(n=5-12)$, where $Rg =Ar$ and Kr, have been used to determine the minimum energy structures and study dynamical properties, such as transitions between liquid- and solid-like structures. A model, developed by Eichenauer and $Le Roy,^{1}$ which predicts the frequency shifts of the $\nu{3}$ band of the infrared-active $SF_{6}$ chromophore, has been used to characterize these clusters. The magnitude of the frequency shift has proved to be a sensitive probe of the distinct catchment regions'' on the potential energy surface which are accessed in the course of a trajectory. Abrupt changes in the frequency shift have been found to be associated with spontaneous iso-energetic isomerizations between structures consisting of a liquid-like monolayer of Ar atoms coating the $SF_{6}$, and others in which a solid-like cap'' of (Ar)n forms on one face of the $SF_{6}$ molecule. It has been observed that ``melting'' is accompanied by a broadening of the simulated $\nu_{3}$ band, behaviour similar to that observed experimentally by Hahn and $Whetten_{2}$ in studies of a benzene chromophore in Ar clusters, Illustrative examples of the use of the infrared-active $\nu_{3}$ band of the $SF_{6}$ chromophore in probing the structure and dynamical properties of some impure rare gas clusters will be presented and discussed.