In recent years, the manipulation of chemistry using hybrid light-matter states called polaritons has generated much research as it combines the speed and efficiency of light with the reactivity and strong interactions of matter. Vibrational polaritons form when a specific vibrational motion of the molecule and photon creates a “spring” that allows them to rapidly exchange energy. This is called strong vibrational coupling (VSC).
While much effort has gone into finding a valid explanation for VSC-modified chemistry and whether vibrational polaritons can alter molecular dynamics, there has been no consensus between theory and experiment.
The question Wei Xiong and Joel Yuen Zhou, chemistry professors at the University of California San Diego, set out to answer was whether polariton modes and dark modes (the molecular byproduct of creating the polariton) both modify chemical reactions. Their article, recently published in Scienceshows unequivocally that chemical reactions occur only with polaritons.
Previous experiments used complex systems that allowed no separation between polaritons and dark modes, making it difficult to differentiate what was happening and impossible to understand what was happening with each mode individually. To remedy this, Xiong used 2D infrared spectroscopy on a simple chemical reaction that was easier to analyze. This allowed his lab to separately excite and track the dynamics of polariton modes and dark modes.
“The big question in the community was whether individual molecules within a cavity could follow their own will,” Xiong said. “In this experiment, we showed that molecules do the same thing over and over on their own, until a polariton ‘leader’ brings them together.”
Xiong explains that this paper sets the stage for continued research into controlling chemical reactions. “If a molecule performs the same reaction over and over again, we’re not controlling it; we’re just observing it,” he said. “Polaritons are a new way to control reactions. We need to consider ways to make molecules act together, synchronized under a photon leader, to amplify their collective power.”
“Theoretically, it’s exciting because we’re not looking at molecules one at a time; we’re looking at them as a many-body system,” said Yuen Zhou. “It’s this idea of collective chemistry and understanding what happens when all the molecules decide to do the same thing. This is the first time we’ve really seen agreement between experimental and theoretical chemistry. The gap between them is closing.”
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Materials provided by University of California – San Diego. Original written by Michelle Franklin. Note: Content can be edited for style and length.