Size-dependence of the electronic spectra of benzene•(N2)n clusters

The evolution of the electronic spectral properties of benzene.(N-2)(n) clusters to these of the corresponding bulk has been studied using a formalism that yields a direct correlation between cluster geometries and spectra. We show here that the calculated spectral shifts of the benzene chromophore as a function of cluster size are in good agreement with experimental values and that, as observed previously, the asymptotic limit of the shifts is considerably smaller in magnitude than the observed bulk values. An examination of the structures of these cryogenic clusters reveals a strong influence of quadrupolar interactions between dinitrogens, interactions that yield a degree of ordering not present in the ostensibly similar benzene (Ar)(n) clusters. These observations suggest a resolution of the problem posed by the apparent lack of asymptotic convergence to bulk spectral shifts, that at the higher temperatures associated with the bulk measurements, higher local dinitrogen densities become possible as the thermal energy exceeds the quadrupolar orientational ordering energy. These higher "solvent" densities would be expected to produce the larger spectral shifts seen in experiments. (C) 1998 American Institute of Physics. [S0021-9606(98)01039-3].