A magnetic trap snares methyl radicals

Physicists have largely succeeded in their quest to tame the atom; in the lab, atoms can now be cooled to their ground translational state, held for minutes, and switched at will between internal states. But the molecule is a wilder beast, its behavior complicated by vibrational and rotational degrees of freedom. Takamasa Momose and his colleagues at the University of British Columbia now report that they’ve trapped a key organic molecule, the methyl radical, in its ground rotational state. It’s only the third molecule with more than two atoms to yield to a laboratory trap. But unlike its predecessors, ammonia and methyl fluoride, and the handful of diatomic molecules that have been caught with electrostatic and optical traps, CH₃ has no electric dipole moment. It does, however, have an unpaired spin. So to capture the molecules as they emerged at sub-kelvin temperatures from a supersonic nozzle, Momose and his coworkers built a meter-long magnetic decelerator comprising 85 solenoid coils, each capable of delivering a 4 T pulse. (The final three coils are illustrated in the image, with magnetic potentials indicated in green.) The pulses are timed so that the paramagnetic CH₃ radicals feel a braking force at each stage. By the time the radicals exit the final stage, they’ve slowed enough to be trapped in the field between two ring-shaped magnets, labeled with red and blue arrows in the figure. The researchers can snare some 50 000 CH₃ radicals in their 1 mm³ cylindrical trap and hold them for more than a second—long enough, they hope, to admit precision measurements of hyperfine transitions, collision cross sections, and reaction rates relevant to the formation of hydrocarbons in interstellar space. (Y. Liu et al., Phys. Rev. Lett. 118, 093201, 2017 .)