Collisions Between Laser-Cooled Atoms and Molecules

Recent work has demonstrated direct laser cooling of diatomic molecules to temperatures of around 5 μK. A promising route to increase the phase-space density is sympathetic cooling with evaporatively cooled atoms. This requires a favourable ratio of elastic to inelastic collisions rates.

We demonstrate a dual species magneto-optical trap of CaF molecules and Rb atoms. After a stage of sub-Doppler cooling and quantum state preparation both species are transferred into a magnetic quadrupole trap. When the molecules are prepared in the first rotationally-excited state, we observe rapid loss due to rotation-changing collisions with the atoms [1]. By contrast, when the molecules are in the ground rotational state we see no inelastic loss. Comparing these measurements to the results of a single-channel loss model based on quantum defect theory, we find a short-range loss parameter close to unity for rotationally excited molecules, but below 0.04 for molecules in the rotational ground state. These results are promising for the prospects of sympathetic cooling of molecules using ultracold atoms.