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Well-resolved vibronic spectra, recorded by resonance two-photon ionization, are analyzed for cold naphthalene cluster species. Spectra for isotopically pure tetramers and both pure and mixed isotopic trimer clusters provide information on resonant (excitonlike) interactions. An analysis of the spectra suggests that the tetramer geometry resembles bulk crystalline naphthalene, whereas the trimer structure minimizes direct contact interactions. Cluster geometry is similar in ground and excited states of these clusters and there is no direct evidence for rapid dynamics in the excited states, nor is there evidence of multiple geometric conformations. Analysis of the isotopically mixed trimers suggests a preference for the HDH conformation, rather than HHD. An empirical weak interaction model provides a successful descripton of the observed spectra. Excitonic splittings are approximately proportional to dipole transition strengths for the vibronically induced spectra, whereas low-intensity allowed origin transitions exhibit splittings that greatly exceed dipolar contributions. Both results indicte that the long-range intermolecular potential exceeds simple estimates. This method of analysis may be applicable to studies of larger clusters, providing new information on size-dependent structure and dynamics.

The (benzene)13 cluster and its isotopic derivatives have been formed in a dilute benzene/helium jet, and investigated by mass-selective R2PI spectroscopy in the region of the B2u ← A1g 000 and 610 transitions. Each band exhibits resolved fine structure, which differs greatly between forbidden (000) and allowed (610) bands. The main features, identified by spectral shift, are proposed to correspond to distinct molecular sites, whose symmetries are deduced by comparing spectra of the allowed and induced vibronic bands. The isotopic substitution of one C6H6 molecule into an otherwise deuterated cluster results in a considerable spectral simplification, due in part to highly nonrandom labelling. The results appear to rule out the crystallographic structure, while being consistent with the compact noncrystallographic structure computed by van de Waal.

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