Nonbulk convergence of solvent spectral shifts in doped molecular clusters

The spectral line of a dopant molecule B in a cluster B·A_n of solvent molecules A has usually been thought to converge to the bulk (gas-to-condensate) shift as n → ∞. Size-resolved electronic spectral measurements on a number of systems do appear to show a rapid and then asymptotic convergence, so that nearly all the shift is recovered by n = 30-50. We discuss here a set of examples - B·(N₂)_n, B·Ar_n, and Al·Ar_n (B = benzene) - in which far from bulk shifts are observed even though apparent asymptotes have been reached. Several models are proposed to explain this effect, and their liabilities are discussed.