A LINEAR DECELERATOR FOR NEUTRAL MOLECULES, identical in principle to a linear accelerator (LINAC) for charged particles, has been demonstrated by researchers in the Netherlands (Gerard Meijer, University of Nijmegen, 011-31-24-365-2277, gerardm@sci.kun.nl), providing a new way to cool molecules to ultralow temperatures. Previous methods for cooling molecules either depend upon the presence of a cold background gas and magnetic fields (Update 393), or they are restricted to those molecules which can be formed by combining already cold trapped atoms. In their demonstration, the researchers constructed a 35-centimeter long "Stark decelerator," containing a succession of 63 pulsed electric fields. The decelerator can slow down any neutral molecule with a permanent dipole moment, i.e., a permanent separation of electric charge within the molecule. This includes any diatomic molecule composed of two different elements (such as NaCl), but also molecules like H2O and NH3. The researchers chose to demonstrate their technique with carbon monoxide (CO). When a pre-cooled mixture of CO in xenon gas entered the linear decelerator, each molecule experienced the Stark effect; at every electric field, their internal energy shifted upward and caused them to lose some kinetic energy. After passing through all 63 electric-field stages, a subset of the CO molecules was slowed down from 225 m/s to 98 m/s, with an equivalent temperature of 30 millikelvin. Additional electric field stages could in principle cool the molecules further. This technique promises to be useful for cold-molecule physics, a field which is "expected to bloom in the next decade," says Meijer. (Bethlem, Berden and Meijer, Phys. Rev. Lett., 23 August 1999.)