Abstract
Laser cooling of molecules is a powerful technique for producing cold, slow beams for precision measurements and quantum control, yet its implementation remains challenging due to molecular complexity. Here, we combine a cryogenic buffer gas beam, an electrostatic hexapole lens, and 2D transverse Doppler laser cooling to produce a bright beam of barium monofluoride (138Ba19F) molecules. We study both numerically and experimentally the laser cooling effect as a function of laser detuning, laser power, laser alignment, and interaction time. We find a scattering rate of 6.1(1.4) × 105 s−1 on the laser cooling transition (14% of the expected maximum) and identify suboptimal dark Zeeman state remixing, suboptimal laser sideband powers and detunings, and a lack of vibrational repump laser intensity as possible causes of such a low rate. Using 3 tuneable lasers with appropriate sidebands and detuning, each molecule scatters approximately 400 photons during 2D laser cooling, limited by the interaction time and scattering rate. Leaks to dark states are less than 10%. Finally, we use the experimental results to benchmark the trajectory simulations to predict the achievable flux 3.5 m downstream for a planned eEDM experiment.
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Data availability
Data are available from the corresponding author upon request.
Code availability
The code we used for analyzing the experimental results and calculating the molecular trajectories is available from the corresponding author upon request.
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Acknowledgements
The NL-eEDM collaboration receives funding (eEDM-166, XL21.074 and VI.C.212.016) from the Dutch Research Council (NWO). We acknowledge the technical support from L. Huisman and O. Böll.
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S.H., H.B., A.B., R.G.E.T., W.U., and L.W. conceived the experiment. J.H. developed the laser system, performed the experiments, analyzed the data, and drafted the first version of the manuscript. J.H., A.T., R.B., and T.F. developed the vacuum system. I.T. and J.H. performed the trajectory simulations using an adapted version of a code originally written by A.T. and H.B. A.T. helped with an early version of the experiment and data analysis. S.H. and H.B. supervised the project. J.H., I.T., A.T., N.B., R.B., H.B., A.B., T.F., S.H., S.J., J.L., M.M., H.M., B.N., E.P., B.S., L.S., R.T., W.U., J.V., and L.W. discussed and approved the final manuscript.
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van Hofslot, J.W.F., Thompson, I.E., Touwen, A. et al. 2D transverse laser cooling of a hexapole focused beam of cold BaF molecules. Commun Phys (2026). https://doi.org/10.1038/s42005-025-02470-x
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DOI: https://doi.org/10.1038/s42005-025-02470-x
