JILA Scientists Demonstrate First Controlled Chemical Reactions of Ultracold Molecules

One of the first-ever images of a molecular gas in which each molecule is in its lowest possible energy state. The molecules are near absolute zero, a temperature at which quantum properties reign. The image—made by detecting the absorption of laser light by the molecules—reveals their spatial distribution, with density indicated by peak height and false color. The fact that such an image can be created indicates the molecular quantum gas is dense enough to enable scientists to observe novel interactions among the molecules. Credit: D. Wang/JILA View hi-resolution image

Physicists at JILA have for
the first time observed chemical reactions near absolute zero,
demonstrating that chemistry is possible at ultralow temperatures and
that reaction rates can be controlled using quantum mechanics, the
peculiar rules of submicroscopic physics. The new results and
techniques, described in the Feb. 12 issue of Science,* will
help scientists understand previously unknown aspects of how molecules
interact, knowledge of fundamental importance to virtually every one of
the physical sciences and engineering.

JILA is a joint institute of the National Institute of
Standards and Technology (NIST) and the University of Colorado at
Boulder. A NIST theorist at the Joint Quantum Institute, a collaborative
venture of NIST and the University of Maryland, also contributed to the
research.

Ultracold molecules are a hot research area because
they may offer more diverse insights and applications than ultracold
atoms, which scientists have deftly manipulated for more than 20 years.
Scientists have long known how to control the internal states of
molecules, such as their rotational and vibrational energy levels. In
addition, the field of quantum chemistry has existed for decades to
study the effects of the quantum behavior of electrons and
nuclei—constituents of molecules. But until now scientists have been
unable to observe direct consequences of quantum mechanical motions of
whole molecules on the chemical reaction process. Creating simple
molecules and chilling them almost to a standstill makes this possible
by presenting a simpler and more placid environment that can reveal
subtle, previously unobserved chemical phenomena.

In conventional chemistry at room temperature,
molecules may collide and react to form different compounds, releasing
heat. In JILA’s ultracold experiments, quantum mechanics reigns and the
molecules spread out as ethereal rippling waves instead of acting as
barbell-like solid particles. They do not collide in the conventional
sense. Rather, as their quantum mechanical wave properties overlap, the
molecules sense each other from as much as 100 times farther apart than
would be expected under ordinary conditions. At this distance the
molecules either scatter from one another or, if quantum conditions are
right, swap atoms. Scientists expect to be able to control long-range
interactions by creating molecules with specific internal states and
“tuning” their reaction energies with electric and magnetic fields.

The JILA team produced a highly dense gas of molecules
consisting of one potassium atom and one rubidium atom at temperatures
of a few hundred billionths of a Kelvin (nanokelvins) above absolute
zero (minus 273 degrees Celsius or minus 459 degrees Fahrenheit). They
found that, although molecules move slowly at ultralow temperatures,
reactions can occur very quickly. However, reactions can be suppressed
using quantum mechanics. For instance, a cloud of molecules in the
lowest-energy electronic, vibrational and rotational states reacts
differently if the nuclear spins of some molecules are flipped. If a
cloud of molecules is divided 50/50 into two different nuclear spin
states, reactions proceed 10 to 100 times faster than if all molecules
possess the same spin state. Thus, by purifying the gas (by preparing
all molecules in the same spin state), scientists can deliberately
suppress reactions.

“We are observing a new fundamental aspect of
chemistry—it gives us a new ‘knob’ to understand and control reactions,”
says NIST physicist Jun Ye, one of the lead researchers on the project.

For more details, see the NIST Feb. 11 news release, “Seeing
the Quantum in Chemistry: JILA Scientists Control Chemical Reactions of
Ultracold Molecules.” [www.nist.gov/public_affairs/releases/ultracold_021110.html]
This research was supported by NIST, the National Science Foundation
and the Department of Energy.

* S. Ospelkaus, K.K. Ni, D. Wang, M.H.G. de Miranda,
B. Neyenhuis, G. Quéméner, P.S. Julienne, J.L. Bohn, D.S. Jin and J.
Ye. Quantum-state controlled chemical reactions of ultracold KRb
molecules. Science. Feb. 12, 2010.

Media Contact: Laura Ost, laura.ost@nist.gov, (303) 497-4880