  Photoassociation of ultracold atoms is a resonant light-assisted collision
process, in which two colliding atoms absorb a photon and form an excited
molecule. Since the first observation about three decades ago, the
photoassociation of ultracold atoms has made a significant impact on the study
of ultracold atoms and molecules. Extending the photoassociation of atoms to
the photoassociation of atom-molecule pairs or molecule-molecule pairs will
offer many new opportunities in the study of precision polyatomic molecular
spectroscopy, formation of ultracold polyatomic molecules, and quantum control
of molecular collisions and reactions. However, the high density of states and
the photoexcitation of the collision complex by the trapping laser make
photoassociation into well-defined quantum states of polyatomic molecules
extremely difficult. Here we report on the observation of photoassociation
resonances in ultracold collisions between $^{23}$Na$^{40}$K molecules and
$^{40}$K atoms. We perform photoassociation in a long-wavelength optical dipole
trap to form deeply bound triatomic molecules in the electronically excited
states. The atom-molecule Feshbach resonance is used to enhance the free-bound
Franck-Condon overlap. The photoassociation into well-defined quantum states of
excited triatomic molecules is identified by observing resonantly enhanced loss
features. These loss features depend on the polarization of the
photoassociation lasers, allowing us to assign the rotational quantum numbers.
The observation of ultracold atom-molecule photoassociation resonances paves
the way toward preparing ground-state triatomic molecules, provides a new
high-resolution spectroscopy technique for polyatomic molecules, and is also
important to atom-molecule Feshbach resonances.
