Sympathetic Cooling of Na$+$ Ions by Ultracold Na Atoms in a Hybrid Trap

Laser cooling atoms to ultracold temperatures has opened a fruitful new regime for atomic physics. Closed-shell atomic ions, such as Na$+$, and nearly all molecular ions lack the optical transitions from the ground state that are required for laser cooling, restricting their use in a variety of experiments: near zero-K reaction studies, cold ion spectroscopy and quantum gates. We have created a hybrid atom-ion trap system to study cooling and reactions of atomic and molecular ions which cannot be laser cooled. It consists of a magneto-optical trap (MOT) for Na, concentric with a linear Paul r.f. ion trap. Recent simulations we have carried out using SIMION 7 [PRA \underline {86}, 033408 (2012)] show that cold MOT atoms may be used to sympathetically cool hot atomic or molecular\_ions to sub-Kelvin temperatures. We found experimental evidence of this: trapped Na$+$ ions exposed to equal mass Na MOT atoms have extended lifetimes when MOT-refrigerated in the Paul trap. Unwanted ions (e.g. Na$_{2}^{+}$ from the MOT) may be quenched with minimal disturbance of the trapped Na$^{+}$ ions.