Spectroscopic signatures of structural aufbau in (benzene)_n; n=7-19

Optical spectra of larger (benzene)_n clusters, n = 7-19, have been investigated in detail using the molecular B_2u ←A _1g0₀⁰ and 6₀¹ transitions. The clusters are formed in a helium jet and are detected mass selectively by two-color resonant two-photon ionization spectroscopy at moderate resolution. The association of spectral line shifts with distinct molecular sites leads to a simple picture of the building up of clusters of nonpolar molecules, as proposed earlier [Easter et al. Chem. Phys. Lett. 157, 277 (1989)]. Coarse band shapes and fingerprintlike fine structure are discussed in terms of the numbers and multiplicities of sites predicted by the icosahedral aufbau sequence, and yield a particularly clear correspondence around B ₁₃, whose quasiicosahedral structure (as determined by minimum-energy simulations) forms the sequence's core. Distinctive multiplet structure observed in the center of the 6₀¹ band for n = 12-15 is attributed to exciton interactions among equivalent sites.