Evidence for the association of triatomic molecules in ultracold 23 Na 40 K +  40 K mixtures.

Ultracold assembly of diatomic molecules has enabled great advances in controlled chemistry, ultracold chemical physics and quantum simulation with molecules 1-3 . 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 them 4,5 . Although ultracold atom-diatomic-molecule Feshbach resonances have been observed recently 6,7 , 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 23 Na 40 K molecules in the rovibrational ground state and 40 K atoms. We apply a radio-frequency pulse to drive the free-bound transition in ultracold mixtures of 23 Na 40 K and 40 K and monitor the loss of 23 Na 40 K 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.