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Vibrational polaritons have shown potential in influencing chemical reactions, but the exact mechanism by which they impact vibrational energy redistribution, crucial for rational polariton chemistry design, remains unclear. In this work, we shed light on this aspect by revealing the role of solvent phonon modes in facilitating the energy relaxation process from the polaritons formed of aT_1umode of W(CO)₆to an IR inactiveE_gmode. Ultrafast dynamic measurements indicate that along with the direct relaxation to the darkT_1umodes, lower polaritons also transition to an intermediate state, which then subsequently relaxes to theT_1umode. We reason that the intermediate state could correspond to the near-in-energy Raman activeE_gmode, which is populated through a phonon scattering process. This proposed mechanism finds support in the observed dependence of the IR-inactive state’s population on the factors influencing phonon density of states, e.g., solvents. The significance of the Raman mode’s involvement emphasizes the importance of non-IR active modes in modifying chemical reactions and ultrafast molecular dynamics.

 

In this study, the “particle in a box” idea, which was broadly developed in semiconductor quantum dot research, was extended into mid-infrared (IR) cavity modes by applying lateral confinement in an optical cavity. The discrete cavity modes hybridized with molecular vibrational modes, resulting in a quartet of polariton states that can support multiple coherence states in the IR regime. We applied tailored pump pulse sequences to selectively prepare these coherences and verified the multi-coherence existence. The simulation based on Lindblad equation showed that because the quartet of polariton states resided in the same cavity, they were specifically robust toward decoherence caused by fluctuations in space. The multiple robust coherences paved the way for entangled states and coherent interactions between cavity polaritons, which would be critical for advancing polariton-based quantum information technology.