Slow Salt

Through laser cooling it's relatively easy to cool atoms to microkelvin temperatures. This method is not useful for molecules, which possess a variety of internal vibrational and rotational motions. By indirect methods, however, stationary samples of molecular vapors have been chilled to mK temperatures by cooling molecules in cold helium or by decelerating polar molecules, or to microkelvin temperatures by welding together pairs of already cooled atoms.

Another cooling technique employs a spinning beam source whose speed cancels the velocity of the molecules emerging from the source. Molecular speeds down to around 60 m/s have been obtained. Now, two physicists at the Universitat Bielefeld (Germany) have produced a beam of potassium-bromine molecules (essentially a kind of salt) with an average molecular speed of 42 m/s; an estimated 7% of the beam travels even slower than 14 m/s (below 1.4K). At this speed, some of the molecules could be loaded into a trap.

The cold KBr molecules are made by sending a beam of K atoms into a counter-propagating beam of HBr molecules where the velocity of both species have to be tuned properly. Within the intersection zone the slow KBr molecules are formed by chemical reaction. There the density of trappable molecules is about two million molecules per cubic centimeter, but the researchers believe this can be increased by a thousandfold.

Besides KBr, beams of other heavy salt molecules can be produced (such as CsI) as well as beams of radicals (reactive molecules with unpaired electrons) such as CaBr and BaI. According to Hansjuergen Loesch (loesch@physik.uni-bielefeld.de), slow molecules are a prerequisite for performing cold chemistry, which would simulate conditions in cold planetary atmospheres or in cold interstellar clouds. If the chemistry is cold enough, new quantum effects might emerge. (Liu and Loesch, Physical Review Letters, 9 March 2007