Focus: The Coolest Molecular Ion Beams
Two research teams have created the coldest molecular ion beams ever, putting molecules in their ground states of rotation and providing improved experimental stand-ins for interstellar gas clouds.
Cooling a beam of molecular ions so that they occupy their lowest-energy quantum state (ground state) would permit high-precision study of their properties and would improve understanding of star-forming gas clouds. Now research teams in Sweden and Germany have accomplished this feat by placing the entire apparatus for trapping and probing molecular ions inside cryogenic cooling systems at temperatures of around 10 degrees above absolute zero. They expect to carry out experiments relevant for astrophysics at a new level of precision.
Reactions between cold atoms, molecules, and ions in interstellar clouds create the material from which stars and planets are made. The states of the molecules involved in these processes can be studied by measuring how the gas absorbs light from more distant stars, but to understand these spectra, researchers need data from laboratory measurements on cold molecules.
[...] Previous efforts to cool molecular ions have mostly used collisions with surrounding atoms of an inert cold gas such as helium. But for reasons that are still not entirely clear, these methods failed to bring the ion temperature below about 20 K.
[...] The sheer technical achievement of cooling the ions to their lowest rotational state is “the most important result of these papers,” says molecular physicist Klavs Hansen of Tianjin University in China. But Hansen warns that “larger ions may require lower temperatures to reach similar rotational states.” However, says Schmidt, the temperatures currently attained are comparable to those in molecular clouds, so the rotational states of any ion they choose to study should still be relevant for astrophysics.
“The development of the cryogenic storage rings is a major advance in instrumentation,” agrees Roland Wester of the University of Innsbruck in Austria, who led previous efforts at collisional cooling. “The long interaction times in an almost completely collision-free environment allow for experiments with unprecedented sensitivity.”
Rotationally Cold OH− Ions in the Cryogenic Electrostatic Ion-Beam Storage Ring DESIREE
H. T. Schmidt, G. Eklund, et al