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Molecules get superchilly reaction: experiments, theory illuminate ultracold quantum chemistry.

Researchers have now been able to stop and start chemical reactions between molecules at temperatures colder than the depths of outer space. And a new theoretical description helps explain the quantum mechanical details of how these reactions happen.

The details, presented March 17, offer glimpses into the burgeoning field of ultracold physics (SN: 12/20/08, p. 22).

Deborah Jin and Jun Ye of the University of Colorado at Boulder and the JILA research center in Boulder led experiments using lasers and electric fields to manipulate reactions between ultracold potassium-rubidium molecules.

"It's a beautiful demonstration of how quantum mechanics works," said chemist Jeremy Hutson of the University of Durham in England. The new studies reveal strange quantum effects "in a very simple regime that's never been explored before."

Jin, Ye and colleagues used lasers to cool the potassium-rubidium molecules, halting nearly all their usual frenetic motion. Held in this chilly "ground state" at a temperature around 200 nanokelvins, the molecules moved incredibly slowly, Ye said. But after a second or so, they started to disappear by twos.

"What's going on here is chemistry," Jin says. The potassium-rubidium molecules can interact with each other to form molecules made up of two potassium atoms and two rubidium atoms, the team reported in the Feb. 12 Science.

At the meeting, Jin and Ye presented new preliminary results showing not only that these chemical reactions occur, but that they also can be sped up or halted. Potassium-rubidium molecules have electric charges at each end-slightly negative at the potassium atom and slightly positive at the rubidium atom. These dipole moments give researchers a way to manipulate the ultracold molecules. When the team turned up an electric field around the molecules, the reaction rate went up dramatically-by a factor of 20 or 30, Jin said.

In other experiments, the researchers showed that the reactions, which happen only when the molecules line up head to tail, could be suppressed. The team used a laser to slice a big glob of ultracold molecules into 20 or so thin, pancakelike shapes. This tight conformation prevented the molecules from lining up head to tail, curbing the reactions. The ability to halt reactions is important, Jin said. "Chemical reactions are great and fun," until an experiment requires the original molecules themselves, she said. "Then those chemical reactions are kind of a problem."

New theoretical results reported at the meeting illuminate how the ultracold molecules find each other in the first place. At temperatures this low, the molecules behave more like diffuse waves than discrete spots, said theorist Paul Julienne of the National Institute of Standards and Technology in Gaithersburg, Md. Julienne and Zbigniew Idziaszek of the University of Warsaw found that long-range effects of these waves, which can reach hundreds of nanometers, influence how the ultracold molecules close in on each other.

Once the molecules are within one nanometer or so, they "react with great certainty," Julienne said. The predicted reaction rates, published March 18 in Physical Review Letters, agree well with the rates observed by Jin and Ye.

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Title Annotation:Matter & Energy
Author:Sanders, Laura
Publication:Science News
Geographic Code:1USA
Date:Apr 10, 2010
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