SOLVATION AND MELTING IN LARGE BENZENE-CENTER-DOT(AR)(N) CLUSTERS - ELECTRONIC SPECTRAL SHIFTS AND LINEWIDTHS

Although there has been considerable interest in solvation processes i n small atomic and molecular clusters, uncertainties in the interpreta tion of spectral probes have made the experimental elucidation of the solvation, and in particular how it relates to bulk solvation, problem atical. We demonstrate here that, through the application of a microsc opic formalism which has the novel feature of accounting for the colle ctive dielectric response of a cluster, the reported spectra of large benzene .(Ar)(n) clusters can be readily understood. Specifically, we show that the apparent lack of convergence of the benzene's absorption spectrum to the corresponding bulk result derives from the dominance of nonwetting cluster structures for large n. Even observed peak multi plicities and individual linewidths may be understood within this form alism if the cluster structures upon which the calculations are based are generated in a nonequilibrium (rather than thermally equilibrated) simulation. Given this detailed understanding of the relationship bet ween spectroscopy and structure, we also can clarify the experimental consequences of the so-called ''melting'' transition in benzene .(Ar)( n) clusters: The spectral signature of the melting is a change in the behavior of the linewidth of the absorption envelope which results fro m a subset, but not all, of the Ar atoms becoming fluid. This descript ion of the melting behavior suggests an important refinement of the co nventional picture of solid-fluid phase coexistence in clusters. (C) 1 996 American Institute of Physics.