Magnetic trapping of calcium monohydride molecules at millikelvin temperatures

Recent advances *RF 1-5* in the magnetic trapping and evaporative cooling of atoms to nanokelvin temperatures have opened important areas of research, such as Bose-Einstein condensation and ultracold atomic collisions. Similarly, the ability to trap and cool molecules should facilitate the study of ultracold molecular physics and collisions [6]; improvements in molecular spectroscopy could be anticipated. Also, ultracold molecules could aid the search for electric dipole moments of elementary particles [7]. But although laser cooling (in the case of alkali metals [1,8,9]) and cryogenic surface thermalization (in the case of hydrogen [10,11]) are currently used to cool some atoms sufficiently to permit their loading into magnetic traps, such techniques are not applicable to molecules, because of the latter's complex internal energy-level structure. (Indeed, most atoms have resisted trapping by these techniques.) We have reported a more general loading technique [12] based on elastic collisions with a cold buffer gas, and have used it to trap atomic chromium and europium [13,14]. Here we apply this technique to magnetically trap a molecular species-calcium monohydride (CaH). We use Zeeman spectroscopy to determine the number of trapped molecules and their temperature, and set upper bounds on the cross-sectional areas of collisional relaxation processes. The technique should be applicable to many paramagnetic molecules and atoms.