Harvard researchers discover that extremely cold temperatures slow chemical reactions at slow speeds, giving them a glimpse of how molecules transform and insights into the quantum world. Credit: Ni Lab / Harvard University
By manipulating ultra-cold molecules in the midst of a chemical reaction, the researchers break an act of molecular disappearance.
In a famous parable, three blind men encounter an elephant for the first time. Each one touches a part (the trunk, the ear or the side) and concludes that the creature is a thick snake, a fan or a wall. This elephant, said Kang-Kuen Ni, is like the quantum world. Scientists can only explore one cell of this vast unknown creature at a time. Now, Ni has revealed a few more to explore.
It all started last December, when she and her team completed a new apparatus that could accomplish the chemical reactions at the lowest temperature of any technology currently available, and then broke and formed the coldest bonds in the history of molecular coupling. But her ultra-cold reactions also unexpectedly slowed down the reaction at a slow speed, giving researchers a real-time view of what happens during a chemical transformation. Now, although the reactions are considered too fast to measure, Ni not only determined the lifespan of that reaction, but solved an ultra-cold mystery in the process.
Using ultra-cold chemistry, Ni, Morris Kahn’s associate professor of chemistry and chemical biology and physics, and her team cooled two potassium-rubidium molecules just above Absolute zero and found the “intermediate”, the space where reagents are transformed into products, they lived for approximately 360 nanoseconds (still millionths of a second, but almost a million times longer than they do for reactions at higher temperatures). “It is not the reagent. It is not the product. It’s something in-between, “said Ni. Seeing that transformation, like touching the side of an elephant, can tell you something new about how molecules work, the foundation of everything.
But they didn’t just watch.
“This thing lives so long that we can now play with it … with light,” said Yu Liu, a graduate student at the Graduate School of Arts and Sciences and first author in his study published in Nature Physics. “Typical complexes, like those from a reaction at room temperature, would not be able to do much because they dissociate into products very quickly.”
Professor Kang-Kuen Ni (center, orange jacket) and Ming-Guang postdoc (right) and Yu Liu (left) measured the lifespan of the coldest reaction in the known universe and, in the process, solved the mystery of how some ultracool molecules disappear. Credit: Kris Snibbe / Harvard Staff Photographer
Like the rays of the Star Trek tractor, lasers can trap and manipulate molecules. In ultra-cold physics, this is the reference method for capturing and controlling atoms, observing them in their quantum ground state, or forcing them to react. But when scientists went from manipulating atoms to manipulating molecules, something strange happened: the molecules began to disappear from view.
“They prepared these molecules, hoping to make many of the applications that promise, for example, to build quantum computers, but instead what they see is loss,” Liu said.
Alkali atoms, like potassium and rubidium Ni and your team’s study, are easy to cool in the ultra-cold regime. In 1997, scientists won a Nobel Prize in Physics for cooling and trapping alkaline atoms in laser light. But molecules are more unstable than atoms: They’re not just sitting there idle, Liu said, they can rotate and vibrate. When trapped together in laser light, the gas molecules collide with each other as expected, but some simply disappeared.
The scientists speculated that the molecular loss was the result of reactions: two molecules came together and instead of splitting in different directions, they transformed into new species. But how?
“What we find in this document answers that question,” Liu said. “What you use to confine the molecules is to kill them.” In other words, it is the fault of the light.
When Liu and Ni used lasers to manipulate that intermediate complex, half of their chemical reaction, they discovered that light forced molecules to abandon their typical reaction path and enter a new one. A couple of molecules, bound together as an intermediate complex, can “photo-excite” rather than go their traditional way, Liu said. Alkaline molecules are particularly susceptible due to how long they live in their intermediate complex.
“Basically, if you want to eliminate the loss,” Liu said, “you must turn off the light. You have to find another way to trap these things. “Magnets, for example, or electric fields can also trap molecules.” But all of these are technically demanding, “Liu said. Light is simply simpler.
“This reaction is, like many other chemical reactions, a kind of universe in itself” – Yu Liu, Ph.D. Physics candidate at the Graduate School of Arts and Sciences
Then Ni doesn’t want to see where these complexes go when they disappear. Certain wavelengths of light (such as the infrared the team used to excite their potassium-rubidium molecules) can create different reaction pathways, but no one knows what wavelengths send molecules to the new formations.
They also plan to explore what the complex looks like at various stages of transformation. “To test its structure,” Liu said, “we can vary the frequency of light and see how the degree of excitation varies. From there, we can find out where the energy levels of this thing are, which informs about its quantum mechanical construction. “
“We hope this serves as a model system,” said Ni, an example of how researchers can explore other low-temperature reactions that do not involve potassium and rubidium.
“This reaction is, like many other chemical reactions, a kind of universe unto itself,” Liu said. With each new observation, the team reveals a small piece of the giant quantum elephant. Since there are an infinite number of chemical reactions in the known universe, there is still a long way to go.
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Reference: “Photoexcitation of long-lived transient intermediates in ultra-cold reactions” by Yu Liu, Ming-Guang Hu, Matthew A. Nichols, David D. Grimes, Tijs Karman, Hua Guo, and Kang-Kuen Ni, July 20, 2020, Physics of nature.
DOI: 10.1038 / s41567-020-0968-8
This research was supported, in part, by the Department of Energy, the David and Lucile Packard Foundation, the Dutch Research Council (NWO), the National Science Foundation, the Department of Defense Army Research Office and the Alexander Foundation von Humboldt.
Other authors of the article are Ming-Guang Hu, Matthew A. Nichols, David Grimes, Tijs Karman, and Hua Guo.
