Isotopic substitution as a probe of cluster structure, aufbau and isomerism in (benzene)_N, N=11-20

Isotopically substituted benzene clusters (dilute C₆H ₆ in C₆D₆) in the N=11-20 range have been formed in a helium jet and interrogated through the C₆H₆ molecular B_2u←A_1g 0₀⁰ and 6₀¹ transitions. The isotopic shift for perdeuterated vs hydrogenated benzene is sufficiently large that the former is spectroscopically transparent in the region of investigation. Line shapes for the singly substituted clusters are remarkably simple in appearance, being dominated by a few sharp lines at large spectral shift. This indicates that the predominant observed absorption for each size derives from the cluster's interior site. Analysis of the data strongly corroborates the icosahedral aufbau hypothesis and, in particular, a shell closing at N=13. Some aspects of the spectral simplification offer compelling new evidence for distinct stable isomers for several cluster sizes.