Effects of Electronic Structure on Molecular Plasmon Dynamics

Collective, coherent excitations in molecules, termed molecular plasmons, can be observed in neutral and charged polycyclic aromatic hydrocarbons (PAHs). Systems in this few-atom limit show behavior strongly dependent on charge state, where the addition or removal of even a single electron dramatically alters electronic and optical properties. Here, we investigate the dynamics of PAHs by studying their excited-state lifetimes in four different charge states: cation, neutral, anion, and dianion. Those characterized by a closed-shell electronic structurethe neutral molecule and the dianionexhibit long-lived, exponentially decaying lifetimes typical of radiative relaxation. In contrast, the open-shell cationic and anionic states exhibit far more rapid multiexponential decay dynamics. This can be attributed to the nonradiative de-excitation of multiple electron–hole pairs in the molecule through molecular plasmon “dephasing” and vibrational relaxation. This study gives insight into the nature of excited states of open- and closed-shell molecules and illuminates the role played by electronic structure in the collective electron dynamics of few-atom plasmonic systems.