Ultracold assembly of diatomic molecules has enabled great advances in controlled chemistry, ultracold chemical physics and quantum simulation with molecules1,2,3. Extending the ultracold association to triatomic molecules will offer many new research opportunities and challenges in these fields. A possible approach is to form triatomic molecules in a mixture of ultracold atoms and diatomic molecules by using a Feshbach resonance between them4,5. Although ultracold atom–diatomic-molecule Feshbach resonances have been observed recently6,7, using these resonances to form triatomic molecules remains challenging. Here we report on evidence of the association of triatomic molecules near the Feshbach resonance between 23Na40K molecules in the rovibrational ground state and 40K atoms. We apply a radio-frequency pulse to drive the free-bound transition in ultracold mixtures of 23Na40K and 40K and monitor the loss of 23Na40K molecules. The association of triatomic molecules manifests itself as an additional loss feature in the radio-frequency spectra, which can be distinguished from the atomic loss feature. The observation that the distance between the association feature and the atomic transition changes with the magnetic field provides strong evidence for the formation of triatomic molecules. The binding energy of the triatomic molecules is estimated from the measurements. Our work contributes to the understanding of the complex ultracold atom–molecule Feshbach resonances and may open up an avenue towards the preparation and control of ultracold triatomic molecules.
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We thank P. Julienne, J. Hutson, J. Ye, B. Gao and J. Doyle for helpful discussions. This work was supported by the National Key R&D Program of China (under grant number 2018YFA0306502), the National Natural Science Foundation of China (under grant numbers 11521063 and 11904355), the Chinese Academy of Sciences, the Anhui Initiative in Quantum Information Technologies, the Shanghai Municipal Science and Technology Major Project (grant number 2019SHZDZX01) and the Shanghai Rising-Star Program (grant number 20QA1410000).
The authors declare no competing interests.
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Extended data figures and tables
After preparation of the 23Na40K molecules, the magnetic field is then ramped to the target value Bt in 3 ms, after which we wait 18 ms for the magnetic field to stabilize. The rf association pulse is applied between 21 ms and 51 ms. The magnetic field is measured by rf spectroscopy. The dashed lines represent the target magnetic field Bt.
Extended Data Fig. 2 Comparison of the rf spectra in the continuous-wave dipole trap and in the intensity-modulated dipole trap.
a, b, The rf spectra measured in the continuous-wave dipole trap. We use two overlapping Gaussian functions to fit the data (red solid line), where the Gaussian function for the atomic loss feature is centred at the atomic transition. Each point represents the average of 6–10 measurements and error bars represent the standard error of the mean. As a comparison, the rf spectra measured in the intensity-modulated optical dipole trap are shown in c and d. It can be seen that the association feature is better resolved in the modulated dipole trap.
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Yang, H., Wang, XY., Su, Z. et al. Evidence for the association of triatomic molecules in ultracold 23Na40K + 40K mixtures. Nature 602, 229–233 (2022). https://doi.org/10.1038/s41586-021-04297-2