  Ultracold assembly of diatomic molecules has enabled great advances in
controlled chemistry, ultracold chemical physics, and quantum simulation with
molecules. 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 the ultracold atom and
diatomic molecule mixture by employing the Feshbach resonance between them.
Although the ultracold atom-diatomic-molecule Feshbach resonances have been
observed recently, utilizing these resonances to form triatomic molecules
remains challenging. Here we report on the evidence of the association of
triatomic molecules near the Feshbach resonances 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 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 binding energy of
triatomic molecule is estimated from the measurement. Our work is helpful to
understand the complex ultracold atom-molecule Feshbach resonance and may open
up an avenue towards the preparation and control of ultracold triatomic
molecules.
