ELECTRONIC ABSORPTION-SPECTRA OF LARGE BENZENE - ARN CLUSTERS

We report the mass resolved resonant two-photon ionization spectra of C6D6 . Ar(n) clusters up to n = 70. Shifts of the benzene S1 <-- S0 6( 0)1 vibronic band were studied as a function of both cluster size and expansion conditions. We find that clusters in different size ranges e xhibit remarkably different spectra, which also depend on expansion co nditions and, hence, cluster internal energy. Below n = 16, spectral f eatures trend toward the blue with increasing size. Above n = 16, all features exhibit weak size dependence over wide size ranges. In the n = 20-40 range, four distinct bands were found, which we suggest could be due to fully enclosed fluxional (-55 cm-1 vs free C6D6), partially enclosed rigid (-43 cm-1), and one-sided rigid (-32 cm-1) or one-sided fluxional (also -32 cm-1, broader) structural types. Above n = 40, on ly one band was definitely identified (at -26 cm-1), which may be due to a one-sided rigid structural type. The trends in spectral shift vs size give no indication of approaching bulk-like solvation of the benz ene. Only one subpopulation between n = 20 and 40 may show similarity to macroscopic benzene-argon solutions.