Solvation and melting in large benzene⋅(Ar)n clusters: Electronic spectral shifts and linewidths

Abstract

Although there has been considerable interest in solvation processes in small atomic and molecular clusters, uncertainties in the interpretation of spectral probes have made the experimental elucidation of the solvation, and in particular how it relates to bulk solvation, problematical. We demonstrate here that, through the application of a microscopic formalism which has the novel feature of accounting for the collective 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 multiplicities and individual linewidths may be understood within this formalism 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 between 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 from a subset, but not all, of the Ar atoms becoming fluid. This description of the melting behavior suggests an important refinement of the conventional picture of solid–fluid phase coexistence in clusters.