Excitonic interactions in naphthalene clusters

Abstract

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 description 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 indicate 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.