Buffer gas cooling of Barium monohydride (BaH)

Buffer gas cooling is a general and widely used technique in which an inert buffer gas, such as Helium is brought down to cryogenic temperatures around 4 K, and is then made to thermalize with a molecule of interest in a cold copper cell. In this rapid thermalization process, both translational and rotational energy in the molecule is carried away in the many elastic collisions with the Helium atoms. In the experiment at ZLab in Columbia, Helium gas flows into the cell via a small pipe, and our molecule of interest, BaH, is introduced via laser ablation of a solid target. A small output aperture at one end of the cell allows the molecules to be swept out of the cell before they get to the walls and freeze, but after the Helium has taken away most of the energy. The resulting stream of molecules is at temperatures less than 4 K, and are moving forward as a group with as much as 20% of the molecules with velocities less than 100 m/s.

Laser cooling molecules

Laser cooling is a proven and effective way to cool (slow) atoms down. The idea is to scatter many of the same color photons off of the atom, opposite to it's direction of motion. This process requires a closed cycling transition, where an atom that is excited by a laser spontaneously emits a photon and returns to its original state, so that the same laser can re-excite it again, many times. Until recently, molecules were not considered for laser cooling since most molecules do not have closed cycling transitions - that is, a molecule may absorb a photon and go to an excited state, but when it emits a photon, it does not return to the original state. In ZLab at Columbia, we chose BaH as a laser cooling candidate because it has a >96% probability to return to the original state when excited. 96% is excellent (as molecules go), but it still means we will lose the molecule after only about 30 photon scattering events. To address this, we add 'repump' lasers, which put the molecules back into the original transition if they happen to fall out. Adding just 2 'repumping' lasers can improve our cycling probability to 99.9999%, allowing us to cycle, on average, up to 90,000 photons! With the head start that buffer gas cooling gave our BaH molecules, laser cooling should be able to slow them down to ultracold temperatures.

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