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Evaporative cooling of the dipolar hydroxyl radical


Atomic physics was revolutionized by the development of forced evaporative cooling, which led directly to the observation of Bose–Einstein condensation1,2, quantum-degenerate Fermi gases3 and ultracold optical lattice simulations of condensed-matter phenomena4. More recently, substantial progress has been made in the production of cold molecular gases5. Their permanent electric dipole moment is expected to generate systems with varied and controllable phases6,7,8, dynamics9,10,11 and chemistry12,13,14. However, although advances have been made15 in both direct cooling and cold-association techniques, evaporative cooling has not been achieved so far. This is due to unfavourable ratios of elastic to inelastic scattering13 and impractically slow thermalization rates in the available trapped species. Here we report the observation of microwave-forced evaporative cooling of neutral hydroxyl (OH) molecules loaded from a Stark-decelerated beam into an extremely high-gradient magnetic quadrupole trap. We demonstrate cooling by at least one order of magnitude in temperature, and a corresponding increase in phase-space density by three orders of magnitude, limited only by the low-temperature sensitivity of our spectroscopic thermometry technique. With evaporative cooling and a sufficiently large initial population, much colder temperatures are possible; even a quantum-degenerate gas of this dipolar radical (or anything else it can sympathetically cool) may be within reach.

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Figure 1: Ground-state structure and scattering theory of OH.
Figure 2: Trap system and inelastic collisions.
Figure 3: Microwave spectra illustrating six different final temperatures.
Figure 4: Scaling relations observed in the evaporative cooling of OH.


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We thank E. Cornell for discussions and B. Baxley for artistic contributions. We acknowledge funding from the NSF Physics Frontier Center, DOE, AFOSR (MURI), DARPA and NIST.

Author information




B.K.S., M.T.H., M.Y. and J.Y. designed and participated in the experiment, and discussed and interpreted the results. B.K.S. ran the day-to-day experiment and collected all the data. G.Q. and J.L.B. constructed the theory. B.K.S. and J.Y. first outlined the manuscript, and B.K.S. and G.Q. wrote the first draft. All authors discussed the results and contributed to the preparation of the manuscript.

Corresponding author

Correspondence to Jun Ye.

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The authors declare no competing financial interests.

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Stuhl, B., Hummon, M., Yeo, M. et al. Evaporative cooling of the dipolar hydroxyl radical. Nature 492, 396–400 (2012).

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