Cavity-Mediated Collective Resonant Suppression of Local Molecular Vibrations.

Recent advances in polaritonic chemistry suggest that chemical reactions can be controlled via collective vibrational strong coupling (VSC) in a cavity. In this fully analytical work, we demonstrate that the collective vibrations of a molecular ensemble under VSC execute a beating with a period inversely proportional to the collective vacuum Rabi splitting. Significantly, this collective beating is imprinted on the local dynamics and resonantly suppresses individual molecular vibrations when a fraction of molecules is vibrationally excited, as in activated complexes formed in chemical reactions. This emergent beating occurs on significantly longer time scales than the individual molecular vibration or the cavity field oscillation period, peaking at the cavity-molecule resonance, consistent with polaritonic chemistry experiments. The cavity mediates an energy exchange between excited and ground-state molecules, affecting the dynamics of the entire ensemble. These findings suggest that the dynamics in polaritonic chemical reactions may not be in full equilibrium. In the ultrastrong coupling regime, we find that the local vibrations are modified by the cavity even at short time scales. Notably, these dynamical effects also extend to isotropic molecular ensembles in our model. Our analytical model offers insights into how collective VSC can dampen local molecular vibrations at resonance, potentially altering chemical reactivity.