Excitonic interaction in the fluorene dimer

Abstract

The fluorene van der Waals dimer exhibits a complex origin spectrum. This region has been studied by resonance two-photon ionization and by fluorescence excitation spectroscopies. The spectra can be interpreted on the basis of intermediate strength exciton coupling, in which the electronic interaction is comparable to the van der Waals vibrational energies. The spectra are reasonably well described by two distorted adiabatic potential surfaces, which correspond to the two excitonic components of the origin system. A single Franck–Condon active intermolecular mode provides a reasonable description of the system, however the potentials have significant cubic and quartic contributions. Non-Born–Oppenheimer nuclear momentum coupling is present and intermodal (IVR) interactions are observed, even for intermolecular modes as low as v=1. The results are remarkably different from prior observations of excitonic structure in other systems, providing a detailed picture of coupling between electronic and intermolecular motion in a van der Waals dimer.