New Platforms for Polariton Reaction Dynamics

Marissa Weichman Princeton University, Department of Chemistry

Polaritons are hybrid light-matter states with unusual properties that arise from strong interactions between a molecular ensemble and the confined electromagnetic field of an optical cavity. Cavity-coupled molecules appear to demonstrate energetics, reactivity, and photochemistry dramatically distinct from their free-space counterparts, but the mechanisms and scope of these phenomena remain open questions. Validating proposed mechanisms for cavity chemistry will require a new body of experimental work directly surveying strongly-coupled reaction trajectories on clean, easily-modeled reactive potentials, working in close partnership with theory. Here, we discuss two new platforms to investigate condensed-phase and gas-phase molecular reaction dynamics under vibrational strong coupling.

In the condensed phase, we have set out to survey cavity-altered reactivity in radical hydrogen-abstraction processes. These reactions have well-characterized potential energy surfaces; they can be initiated with photolysis and tracked directly on ultrafast timescales; they are exothermic and proceed rapidly so dynamical signatures are not washed out; and they are accessible to theory, enabling detailed interpretation of reaction pathways. We run our reactions in dichroic microcavities that permit vibrational strong coupling in the infrared in combination with broadband optical access in the visible and ultraviolet. We are using ultrafast transient absorption measurements to examine intracavity reaction rates with the goal of pinpointing precisely how these trajectories may be influenced by strong light-matter interactions.

We will also discuss our recent demonstration of gas-phase molecular polaritons. While polaritons are now well-established in solution-phase and solid-state samples, they have not yet been reported in isolated gas-phase molecules, where attaining sufficiently strong light-matter interactions is a challenge. We show that the strong-coupling regime can be accessed in the gas phase at low temperatures where molecules are found in only a few lowest-energy quantum states and their absorption linewidths are narrow. We have built an apparatus that combines a cryogenic buffer gas cell with a feedback-stabilized optical cavity to reach this regime. This new infrastructure allows us to cavity-couple individual rotational-vibrational states, access a range of coupling strengths and detunings from resonance, and tune both molecular and cavity linewidths. We expect that this platform will enable surveys of cavity-altered molecular reactivity, dynamics, and photophysics with quantum-state-specificity and without the complications of solution-phase environment.