Ultracold molecules form inside superatom

A strange form of matter appears when researchers cool gaseous clouds of certain atoms to nearly absolute zero. The ultracold atoms all fall into the same quantum state, becoming a sort of superatom called a Bose-Einstein condensate.

Since they made the first such superatoms almost 5 years ago (SN: 7/15/95, p. 36), physicists have wondered if they could also make condensates of molecules. Molecular condensates would offer new ways of studying chemical reactions and molecular properties, scientists predict.

A group of Texas researchers has now demonstrated a major step toward molecular condensates—making some of the atoms within a condensate hitch up to form molecules. The team, led by Daniel J. Heinzen of the University of Texas at Austin, used laser beams to combine rubidium-87 atoms into pairs.

“This is another great milestone in the area of research with cold molecules,” says John M. Doyle of Harvard University, who produced the first trapped, cold molecules nearly 2 years ago (SN: 5/30/98, p. 342). Those molecules weren’t associated with a condensate.

Besides making two-atom rubidium molecules, the researchers also measured with extraordinary accuracy the energy holding the molecules together. In the Feb. 11 Science, the Texas team reports a 10,000-fold improvement over previous similar measurements on molecules made from cold atoms not in a Bose-Einstein condensate.

Starting with an atomic condensate at about 100 nanokelvins, the Texas physicists made “the coldest molecules around, perhaps the coldest molecules in the universe,” says Paul S. Julienne of the National Institute of Standards and Technology (NIST) in Gaithersburg, Md.

That exceptional frigidity made possible the measurement’s great accuracy. Ordinarily, molecular motions slightly blur measured properties. These low-temperature champions neither rotate nor zip around but still vibrate. Only weakly bonded to each other, the atoms undergo far-ranging oscillations, like agitated weights connected by a stretchy spring.

The researchers succeeded in converting only 1 percent of the 140,000 to 300,000 condensate atoms into molecules. Moreover, because of their weak bonds and collisions with condensate atoms, the molecules disintegrate back into atoms after 100 to 450 microseconds, the experimenters found.

Such difficulties suggest that despite the successes of the new experiment, creating a largely molecular condensate may yet prove to be a pipe dream, Julienne and the Texas researchers agree. Julienne and NIST’s Carl J. Williams coauthored a commentary on the new findings in the same issue of Science.

A better understanding of interactions between condensate atoms and molecules may pave the way to bigger molecular populations in condensates, says Texas team member Roahn Wynar. Toward that end, the group has begun measuring properties of atom-versus-molecule collisions in condensates. The researchers also plan to adjust their apparatus to yield molecules in less excited, more stable states.

“There is a lot of work to be done still,” Wynar says.