Harvard Scientists Illuminate Light’s Surprising Impact on Ultra-Cold Chemistry

Harvard researchers uncovered the surprising role of light in ultra-cold chemical reactions, according to an article published last month in the scientific journal Nature Physics.

Chemistry and Chemical Biology and Physics professor Kang-Kuen Ni and her lab, who conducted the coldest chemical reaction in the known universe last December, discovered that laser beams — commonly used in ultra-cold chemistry to stabilize reaction molecules — were responsible for the loss of molecules from the reaction, a phenomenon known as two-body loss.

Yu Liu, the first author of the study and a graduate student in the Ni lab, said light was a “metaphorical beaker” often used to confine molecules in ultra-cold chemistry.

“We're not really confining the molecules that we're examining in physical objects like typical chemists would do in test tubes and beakers, since if you put something that's only a billionth of a degree above absolute zero in any physical object that's not at that temperature, it will quickly heat up the whole thing and your reaction is toast,” Liu said. “The nice thing about light is that it won't heat up the molecules.”

Liu said researchers realized that light was “doing something bad to the reaction,” causing some of the reaction molecules of ultra-cold potassium and rubidium to simply vanish and not form the expected products.

“When things are cold, microscopically it means atoms and molecules involved are moving very slowly, and that gives us a better handle on controlling them and interrogating them,” Liu said. “But it turns out in this case, as two reacting potassium-rubidium molecules collide, they form this very long-lived complex that lasts for about 360 nanoseconds, and during this time there is a chance for the very light that's confining these molecules to perturb them.”

The researchers found evidence that light was a “steering agent,” which altered the fate of the reaction intermediates by exciting the molecules to deviate from the normal reaction pathway and “leave a very small amount of normal product formation left,” per Liu.

“Straightforward evidence for light doing something is that as we vary the intensity of the light, we see that the reaction can be turned on and off essentially,” Liu said. “So if you don't have light, if you're reacting in absence of light, then you're going to see the products coming out no problem, just as they normally would.”

While the researchers have not yet decided upon a new method for confining the molecules that will prevent molecular loss — as other currently available methods are still very technically demanding — Ni said this latest finding is an important step on the path to understanding chemical reactions on a quantum scale.

“The project is being built layer by layer, and the more that we know, the more we are able to control the system and then build up and do the next thing,” Ni said. “Just as we now know that light is a problem that we need to address, our goal is to step-by-step understand our system really well so we can realize a quantum mechanical explanation of how chemical transformation from reactants to products occurs.”

Ni also noted that while her lab has specifically focused on the reaction between potassium and rubidium molecules, she believes discoveries made by observing this reaction will be widely applicable or biologically-relevant chemical reactions, as well as fundamental reactions in quantum computing and cryptography.

“The quantum mechanics theory we are developing, in principle, will be able to be applied to other systems, so if we can understand this reaction really well, we should also learn something universal,” Ni said. “That is really rewarding.”

—Staff writer Meera S. Nair can be reached at meera.nair@thecrimson.com.