Ultracold Molecules Tamed
In high school chemistry Bunsen burners are used to prompt species into combinations. For ultracold chemistry, carried out at temperatures a billion times colder than room temperature and a million times colder than interstellar space, more targeted methods are necessary, especially if molecules and not atoms are the reactants.
Because much new science is expected from those reactions, ultracold molecular chemistry is now hot stuff. Accordingly, Paul Julienne, a fellow of the Joint Quantum Institute (JQI), along with Goulven Quemener, who works at the Joint Institute for Laboratory Astrophysics (JILA), have written a comprehensive summary of the subject in the journal Chemical Reviews (*). Also, in this week’s issue of Nature Julienne reviews a significant experiment reaching record new temperatures and densities for a chemical species---the hydroxyl radical---with chemical significance in the real world using evaporative cooling (**).
Molecules are much harder to cool than atoms. That’s because in addition to their translational motion, molecules have lots of internal motions, such as vibrations or rotations, which are difficult to cool simultaneously.
Chilling atoms or molecules reduces the Doppler fuzziness that accompanies their light emissions. But another major reason to go to very cold temperatures is that in a quantum sense---atoms and molecules being considered as waves, which are spread-out things---the wave-like size of each molecule can be a hundred or thousand times larger than the range of conventional chemical forces. This allows the molecules’ interactions to be influenced by very tiny electric and magnetic fields imposed from outside.
Julienne and Quemener, in the review article, provide a handy summary of experimental and theoretical work so far with alkali atoms ensconced in polar molecules, molecules which, though neutral, possess a net electric dipole moment. This allows them to be controlled by applied electric fields.
A meeting on exactly this topic will be held at the Kavli Institute for Theoretical Physics in Santa Barbara in January 2013.
Julienne’s Nature essay looks at a very different molecule, OH, one with many real-world applications. Called the “hydroxyl radical,” OH is a very important atmospheric and astrophysical chemical species, a free radical implicated in numerous reactions. It plays a role in chemistry as a simple fundamental species in chemical reactions. Julienne summarizes a paper in the same issue of Nature by researchers at JILA, which is operated by the National Institute of Standards and Technology and the University of Colorado. Jun Ye and his colleagues report great advances in cooling OH---resulting in a tenfold decrease in temperature (down to a few mK) and a thousand-fold increase in phase space density---the number of molecules per quantum (de Broglie) wavelength.
The JILA scientists achieved their colder, denser OH swarm by developing a new form of evaporative cooling, a process in which (as in blowing upon a hot cup of coffee) the warmest atoms or molecules are encouraged to depart. Evaporative cooling doesn’t work as well for molecules as for atoms because a lot of molecule’s energy is stored inside, and the energy is released and heats the molecules when they collide.
With OH, however, the evaporative process could be rescued. Because OH is polar it can be manipulated by electric fields; because it is paramagnetic it can be manipulated magnetically. It could be trapped in a quantum state where an inter-molecular repulsive force serves to keep inelastic (energy-transferring) reactions from taking place among the OH molecules. “Many of them remain trapped long enough for a ‘microwave knife’ to slice away the warmer molecules,” said Julienne, thus allowing cooling to proceed.
Because the JILA experiment can foster a relatively high phase space density of molecules in their trap, Julienne suspects that they can soon achieve much colder temperatures.
(*) “Ultracold Molecules under Control!” by Goulven Quéméner and Paul S. Julienne, in Chemical Review.
(**) Paul Julienne, Nature, 20 December 2012, “Cool Molecules,” commenting on the paper “Evaporative cooling of the dipolar radical OH,” by Stuhl et al.
The Joint Quantum Institute is operated jointly by the National Institute of Standards and Technology in Gaithersburg, MD and the University of Maryland in College Park.