In recent years, manipulation of chemistry with hybrid states of light called polaritons has generated much research because it combines the speed and efficiency of light with the reactivity and strong interactions of matter. Vibrational polaritons form when a certain vibrational motion of the particle and photon creates a “spring” that allows them to rapidly exchange energy. This is called vibratory vibratory coupling (VSC).
Although much effort has been made to find a sound explanation for VSC-modified chemistry and whether vibrational polaritons can alter molecular dynamics, consensus between theory and experiment has been lacking.
The question that chemistry professors Wei Xiong and Joel Yuen Zhou at UCSD sought to answer was whether Polariton patterns and dark modes (the molecular byproduct of Polariton formation) modulate chemical reactions. Their recently published paper in Sciencesunequivocally showing that chemical reactions occur only with polaritons.
Previous experiments used complex systems that did not allow any separation between the polaritons and the dark modes, making it difficult to distinguish what was happening and impossible to understand what happened with either mode separately. To remedy this, Xiong used two-dimensional infrared spectroscopy on a simple chemical reaction that was easy to analyze. This allowed his lab to excite and track the dynamics of the polaritons and dark modes.
“The big question in the community was whether individual molecules inside a cavity could follow their will,” Xiong said. “In this experiment, we showed that the particles do the same thing over and over on their own, until a polariton leader brings them together.”
Xiong explains that this paper lays a foundation for continued research into the control of chemical reactions. “If a molecule performs the same reaction over and over again, we don’t control it; we just observe it,” he said. “Polaritons are a new way to control interactions. We need to think of ways to get the particles to work together, in sync under the photon leader, to amplify their collective force.”
“Theoretically, this is exciting because we’re not looking at molecules one by one; we’re looking at them as a multi-body system,” Yuen Zhou said. “It’s the idea of collective chemistry and understanding what happens when all 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 narrowing.”