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A Cold and Slow Molecular Beam
Due to the rich internal structures and the long range, anisotropic, and tunable interaction of polar molecules, increasingly efforts have been devoted to producing cold polar molecules for studying quantum simulation, cold, controlled chemistry, and precision measurements. We reported producing a cold, slow calcium monohydride molecular beam using a two-stage cryogenic buffer gas cell. This is an alternative to a theoretical Zeeman slower for molecules. The CaH radicals are produced by laser ablation in the first stage cell, where a sufficient helium buffer gas density can be maintained for cooling molecules. The exit aperture of the first cell is chosen to be big enough to efficiently extract the molecules out with the flowing buffer gas. Instead of directly emitting the molecules produced from such a single stage cell into vacuum, where the molecular velocity can be accelerated to the moving velocity of the light He buffer gas due to multiple He-molecule collisions near the aperture, a second cell (or slowing cell) is attached to the first cell. The buffer gas density in the second cell is roughly an order of magnitude lower than that of the first cell, such that molecules can experience a few collisions and then slow down. In addition, the undesirable He-CaH collisions in the beam, which can cause velocity increase, can be reduced. The final exit aperture of the second cell is covered with a piece of copper mesh. The presence of mesh can scatter He atoms back to the second cell and leads to He-CaH collisions which are against the flow direction for additional slowing. The resulting CaH beams are created in two regimes. In one regime, a modestly boosted beam has a forward velocity of vf = 65 m/s (Fig.1 (b)), a narrow velocity spread, and a flux of 109 molecules per pulse. In the other regime, our slowest beam has a forward velocity of vf = 40 m/s, a longitudinal temperature of 3.6 K (Fig. 1 (a)), and a flux of 5 x 108 molecules per pulse. Such a slow molecular beam opens up the possibility of direct loading of molecules into a trap and may be an ideal starting point for direct laser cooling of molecules.