Cold molecules make long-awaited debut.

A dozen years have passed since physicists first trapped ultracold atoms, paving the way for atomic lasers and other advances and netting three cold-atom pioneers the 1997 Nobel Prize in Physics (SN: 10/25/97, p. 263).

All the while, researchers have dreamed of extending this rewarding means of probing atomic structure and behavior to molecules. But the lasers used to cool and trap atoms can't readily put out beams at enough different frequencies to subdue the wide variety of vibrations and energy levels that more complex particles possess.

On May 29, however, a team led by John M. Doyle of Harvard University reported the first evidence of molecular trapping at temperatures near absolute zero. His announcement followed hard on the heels of a report in the May 18 Physical Review Letters from Pierre Pillet and his colleagues at Laboratoire Aime Cotton near Paris describing the first detection of stable--yet untrapped--cold molecules made from cold atoms.

"The big thrill at this point is that everybody who wants to trap molecules looks at [the French group's findings] as a step closer," says Paul D. Lett of the National Institute of Standards and Technology in Gaithersburg, Md.

Lett also described the Harvard advance as "great progress, because people have wanted especially to see some general technique like this where they don't have to build a special laser system for each molecule."

While neither team has yet delivered cold molecules that are pure and confined enough for thoroughly studying their properties, both say a new era of cold molecule physics and chemistry is dawning. "We're at the birth of entirely new areas of working with cold molecules," says Doyle.

New insights into chemical reactions may arise from the study of cold, slow molecular collisions. Also on the horizon may be improved understanding of molecular energy states and, possibly, molecular lasers and Bose-Einstein condensates--collections of ultracold particles all in the same quantum state (SN: 7/15/95, p. 36; 5/25/96, p. 327).

Pillet's group was carrying out the well-established practice of photoassociating, or briefly linking pairs of atoms--cold, trapped cesium, in this case--through interaction with a photon, when they discovered frigid cesium molecules tumbling from the bottom of their trap.

"This was a little bit unexpected," says Pillet, but welcome because the molecules lasted milliseconds, signifying that they were the stable variety sought by physicists. The French group is now attempting to build a type of laser device, known as a dipole trap, to bag their quarry.

Meanwhile, Doyle's team presented spectroscopic data indicating successful trapping of more than a billion molecules of the simple compound calcium monohydride, cooled to 350 millikelvins. Reporting at a meeting in Santa Fe, N.M., of the Division of Atomic, Molecular, and Optical Physics of the American Physical Society, Doyle and his colleagues said they have dispensed with lasers altogether for trapping. Instead, they inject relatively warm molecules into frigid helium gas within a magnetic well that captures the slightly magnetically polarized molecules. Next, they plan to remove the helium and any impurities.