&Bullet; physics 14, p39
Laser cooling can cool molecules down to a few microkelvins, but some experiments require even colder temperatures. One way that researchers suggest to achieve these temperatures is to start with laser-cooled molecules and then remove even more energy through elastic collisions with ultra-cold atoms. However, some atom-molecule collisions are inelastic and should throw molecules out of the trap. Now Sarunas Jurgilas and colleagues from Imperial College London and Durham University, UK, have created such a mixture of laser-cooled molecules and atoms and determined the situations in which these inelastic collision losses occur  .
The team loaded laser-cooled rubidium atoms and calcium monofluoride (CaF) molecules into a magnetic trap and then measured the number of each species that remained a few seconds later. Comparison of these measurements with traps containing only CaF molecules revealed the rate at which molecules escaped from the trap due to interactions with rubidium atoms. Two cases shed light on the mechanism behind these losses: In one experiment, the CaF molecules started in the rotating ground state; in another they started in a rotationally excited state. For the excited state, the rate of loss was equivalent to a molecule escaping every time it approached an atom. For the basic state the loss rate was at least 10 times lower. The result implies that the loss is due to inelastic collisions, in which molecules exchange rotational energy for translational kinetic energy. Controlling the rotational degrees of freedom of molecules can be key to avoiding such collisions.
In the case of atom-molecule mixtures, where elastic collisions are 100 times higher than inelastic collisions, the collision cooling should work well. This cooling technique could improve basic physics tests based on precise measurements of molecular energies and enable new skills in quantum computation and simulation.
Rachel Berkowitz is Corresponding Editor for physics based in Vancouver, Canada.
- S. Jurgilas et al., “Collisions between ultra-cold molecules and atoms in a magnetic trap” Phys. Rev. Lett.126153401 (2021).