New insight into experimental probes of cluster melting

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Abstract
Experiments are now appearing which attempt to probe melting in small clusters, a notable example of these being the recent studies of benzene–Arn clusters by Hahn and Whetten [Phys. Rev. Lett. 6 1, 1190 (1988)]. We report a study of the dynamics of these same benzene–Arn clusters which seeks to clarify further the nature of ‘‘phase transitions’’ in small systems. The techniques used here, involving an instantaneous normal mode analysis based on the results of Monte Carlo calculations, have been shown previously to yield a picture of argon cluster melting which is more complete than the one which emerges from a mere enumeration of low‐energy structures. Although the bare argon clusters are found to undergo dynamical transitions as the cluster temperature is increased, these transitions are inhibited by the presence of an embedded benzene molecule, which provides a template for ordering of the argon atoms. The calculations also suggest a possible explanation for the doubly peaked spectra observed in the experiments of Hahn and Whetten: the benzene may be found in two general types of bonding environments, either surrounded by or on surface of the Arn cluster. This occurrence is consistent with the finding of Bösiger, Knochenmuss, and Leutwyler [Phys. Rev. Lett. 6 2, 3058 (1989)] that the solute–solvent interaction in clusters can be either wetting or nonwetting. We discuss how the evidence necessary to resolve such issues can be derived from an analysis of cluster dynamics.