Spectra of isotopically mixed benzene trimers

Isotopically mixed jets of benzene produced important new detailed structural results from the various possible isotopic benzene trimers in a supersonic jet. From different isotopic spectral shifts, a detailed and consistent model of the interaction between the three benzene molecules can be inferred. The splitting of the 0--0 transition of isotopically mixed trimers can be predicted from this model. This model is consistent with a ''zig--zag'' structure for the trimer, not a cyclic structure. In contrast to the dimer spectra, van der Waals modes were observed. We also discovered an important, new exciton splitting in the van der Waals modes. This splitting demonstrates for the first time that the van der Waals modes are uncoupled from molecular vibrations. Furthermore, it is a sensitive indicator of the identity of the molecules so coupled. The spectra of higher benzene clusters (dimer to pentamer) have also been measured