Ultracold Reactions Probe the Frontiers of Quantum Chemistry

Less than a millionth of a degree kelvin above absolute zero, chemical reactions can still proceed rapidly
Potassium-rubidium molecules

G. Kuebler/JILA

A new study shows that molecules cooled to have near-negligible collisional motion can still react chemically with one another. At just a few hundred nanokelvins above absolute zero, the researchers could even change the speed of the chemical reaction by tweaking the molecules' quantum states, paving the way for highly controlled chemistry using the tools of physics. (A nanokelvin is one billionth of a kelvin.)

The study appears in the February 12 issue of Science, authored by scientists from two institutes affiliated with the National Institute of Standards and Technology (NIST): JILA, run jointly by NIST and the University of Colorado at Boulder; and the Joint Quantum Institute, a partnership between NIST and the University of Maryland, College Park.

In 2008 a group including many of the same researchers announced the creation of a dense gas of potassium–rubidium (KRb) molecules at a few hundred nanokelvins. Now that ultracold gas has been shown to decay through a heat-releasing chemical reaction as its molecules interact through the phenomenon of quantum tunneling, in which particles skip over classical barriers. In this case the blockade is a so-called momentum barrier between two identical molecules experiencing mutual repulsion.

At ultracold temperatures, classical physical conceptions of molecules become less useful than quantum-mechanical ones, says Jun Ye, a JILA physicist and study co-author. "They're so cold that you can no longer think of them as a ping-pong–ball kind of object," Ye says. "They're really quantum-mechanical waves." Those waves can overlap at relatively great distances, leading to remote interactions between the molecules. "As soon as they feel each other's presence, when their wave function starts to overlap, very interesting things start to happen," Ye says. In this case the gas molecules swap atoms to form K2 and Rb2 molecules, which then escape.

Shifting the starting conditions in the potassium–rubidium gas highlighted the quantum-mechanical nature of the chemical reactions. When all the molecules were set to the same initial quantum state, the gas decayed over several seconds. But when the molecules were prepared in different states—specifically, in a heterogeneous mix of spins—the reaction proceeded 10 to 100 times faster. The difference is a logical extension of the Pauli exclusion principle: Identical molecules repel one another to avoid occupying the same place simultaneously.

"In order for them to approach each other, they would have to overcome this momentum barrier," Ye says. "Tunneling is really what is happening; they're tunneling through this angular-momentum barrier." Distinguishable molecules—those with different spins—are freer to cozy up.

Jeremy Hutson, a chemistry professor at Durham University in England who wrote an accompanying commentary for Science on the research, says that the fine-scale physical manipulation possible in the ultracold regime provides an accompanying level of chemical control. "I think it's the selectivity that's remarkable under these circumstances," Hutson says. "You can make a change as tiny as flipping a singular nuclear spin and completely change the course of the reaction."

Hutson says that it may soon be possible to act on a whole population of atoms or molecules at once. "The strength of this general field, this ultracold chemistry, if you like, is that in the future it is likely to be possible to carry out chemical reactions on entire samples of molecules in a coherent and controlled way," he says.

But now that the JILA and Joint Quantum Institute researchers have shown how readily chemistry can proceed at ultracold temperatures, Ye wants to figure out how to nip it in the bud to extend the lifetime of the potassium–rubidium gas, which at present begins to chemically degrade in about a second. "The goal for us is to be able to suppress these reactions," Ye says. "Now that we can understand and control them, the next step is to really learn how to eliminate them."

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