Vibrations and the passage through a diabolic point in the anthracene dimer

Conical intersections (CIs), or diabolic points, play a critical role in fast electronic energy relaxation in aromatic molecules and their aggregates. While it is recognized that vibronic coupling drives the dynamics through CIs, there is a paucity of experimental information on the effect of vibrations on relaxation in molecular aggregates. Here, we describe experiments on the ultrafast electronic energy relaxation in the isolated anthracene dimer-an archetype of a π-stacked aromatic system relevant to intermolecular photon-driven processes such as those in optoelectronics. The combination of a molecular beam with femtosecond, tunable laser spectroscopy allows for both the necessary spectral and time resolution to investigate the relaxation of different vibrational levels in the excited state of the anthracene dimer. In general, our experimental observations corroborate predictions of the lifetimes of initial excited states in aromatic dimers. More specifically, we evaluate our results using time-dependent density functional theory, which identifies potential electronic relaxation pathways via CIs and supports multiple phenomenological interpretations for the observed lifetimes of different vibrational states in the electronically excited state manifold.