Collective Rabi-Driven Vibrational Activation in Molecular Polaritons.

Molecular polaritons arise from electronic or vibrational strong coupling (ESC, VSC) with confined electromagnetic fields. While these have been widely studied, the influence of electron-nuclear dynamics in driven cavities remains largely unknown. Here, we report a previously unrecognized mechanism of vibrational activation that emerges under collective ESC in driven optical cavities. Using simulations that self-consistently combine Maxwell's equations with quantum molecular dynamics, we show that collective electronic Rabi oscillations coherently drive nuclear motion. This effect is captured using both vibrational wave packet dynamics in a minimal two-level model and atomistic simulations based on time-dependent density-functional tight-binding theory. Vibrational activation depends nonmonotonically on the Rabi frequency and is maximized when the collective polaritonic splitting resonates with a molecular vibrational mode. The mechanism exhibits features consistent with a stimulated Raman-like relaxation mechanism. Our predictions are robust under realistic cavity conditions and provide the conditions in which they could be verified experimentally.