Making slow salt

With their various internal vibrational and rotational motions, molecules are difficult to cool. Even so, millikelvin temperatures have been reached by using liquid helium for molecular vapors, and by decelerating polar molecules; microkelvin temperatures are obtained by welding together pairs of cooled atoms. A mechanical technique, using a spinning beam source whose speed cancels the velocity of the emerging molecules, has obtained speeds down to around 60 m/s. With a new kinematic technique, two physicists at the University of Bielefeld in Germany have now produced a beam of potassium–bromine salt molecules with an average molecular speed of 42 m/s; an estimated 7% of the beam travels slower than 14 m/s, corresponding to a temperature below 1.4 K. At that 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 counterpropagating beam of HBr molecules. With the beam velocities carefully tuned, chemical reactions produce the KBr molecules with a very small center-of-mass velocity. Other heavy salt molecules and radicals can also be produced this way, according to researcher Hansjürgen Loesch. Slow molecules are a prerequisite for performing cold chemistry, which could simulate conditions in cold planetary atmospheres or interstellar clouds. If the chemistry is cold enough, new quantum effects might emerge. (N.-N. Liu, H. Loesch, Phys. Rev. Lett. 98 , 103002, 2007 http://dx.doi.org/10.1103/PhysRevLett.98.103002 .)