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Physical Chemistry

Molecules Cooled To Near Zero

Physics: Supercooling, first done in atoms, now has been done to hydroxyl radicals, enabling quantum chemistry

by Elizabeth K. Wilson
December 24, 2012 | APPEARED IN VOLUME 90, ISSUE 52

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Credit: Nature
Controlled by microwave pulses and electric fields, trapped hydroxyl radicals are progressively cooled, as higher energy molecules are allowed to escape.
09052-notw4-Figure2cxd.jpg
Credit: Nature
Controlled by microwave pulses and electric fields, trapped hydroxyl radicals are progressively cooled, as higher energy molecules are allowed to escape.

The creation of the first Bose-Einstein condensate—a supercooled collection of atoms displaying single-particle quantum behavior—won the Nobel Prize in Physics in 2001. Now, scientists have in sight the prospect of taking entire molecules to that level of coldness (Nature, DOI: 10.1038/nature11718).

The unprecedented achievement of “evaporative cooling” of molecules, in this case hydroxyl radicals, means that scientists could study quantum effects in chemistry, says physicist Paul S. Julienne at the University of Maryland and the National Institute of Standards & Technology in a commentary about the work. For example, scientists may eventually be able to control chemical reactions by ­changing the molecules’ quantum spin states.

Physicist Jun Ye, graduate student Benjamin K. Stuhl, and colleagues at JILA—a joint institute between NIST and the University of Colorado, Boulder—used evaporative cooling to bring approximately 1 million hydroxyl radicals down to 5 mK.

Both Julienne and Wolfgang Ketterle, a physics professor at Massachusetts Institute of Technology who shared the 2001 Nobel Prize, say they expect that scientists will soon reach even colder temperatures, perhaps even around 1 µK, where Bose-Einstein behavior comes into play.

Unlike atoms, molecules vibrate and rotate, their inelastic collisions creating a daunting interplay of quantum energy states that interfere with cooling. Although scientists have made advances in cooling molecular gases, the evaporative cooling technique needed to reach temperatures at which Bose-Einstein condensates form was viewed as prohibitively complex.

The researchers broke that barrier by designing a magnetic trap, in which they bombard the molecules with microwave pulses and electric fields, select molecules with higher energies, and allow them to escape the trap.

In this progressive process, which Ketterle calls “really creative,” ever-cooler molecules remained inside the trap until a pool of the supercool molecules remained at the bottom. The authors say the cooling in this experiment was limited only by the sensitivity of their instruments.

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