Observation of Photoassociation Resonances in Ultracold Atom-Molecule Collisions

Jin Cao, Bo-Yuan Wang, Huan Yang, Zhi-Jie Fan, Zhen Su, Jun Rui, Bo Zhao, and Jian-Wei Pan

Phys. Rev. Lett. 132, 093403 – Published 1 March 2024

Abstract

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 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 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.

Received 10 August 2023
Revised 1 January 2024
Accepted 8 February 2024

DOI:https://doi.org/10.1103/PhysRevLett.132.093403

Physics Subject Headings (PhySH)

Cold and ultracold molecules Photoassociation Ultracold collisions

Ultracold gases

Atomic, Molecular & Optical

Authors & Affiliations

Jin Cao^1,2,*, Bo-Yuan Wang^1,2,*, Huan Yang ^1,2,3,*, Zhi-Jie Fan^1,2, Zhen Su^1,2, Jun Rui ^1,2,3, Bo Zhao ^1,2,3, and Jian-Wei Pan^1,2,3

¹Hefei National Research Center for Physical Sciences at the Microscale and School of Physical Sciences, University of Science and Technology of China, Hefei 230026, China
²Shanghai Research Center for Quantum Science and CAS Center for Excellence in Quantum Information and Quantum Physics, University of Science and Technology of China, Shanghai 201315, China
³Hefei National Laboratory, University of Science and Technology of China, Hefei 230088, China

^*These authors contributed equally to this work.

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Vol. 132, Iss. 9 — 1 March 2024

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Images

Figure 1
Illustration of photoassociation in ultracold collisions between ${}^{23}{Na} {}^{40}K$ molecules and ${}^{40}K$ atoms. The ultracold ${}^{23}{Na} {}^{40}K$ molecule and ${}^{40}K$ atom absorb a photon and form a triatomic molecule in the electronically excited state if the laser frequency is in resonance with a free-bound transition. To suppress the photoexcitation of the collision complex by the trapping laser and to avoid the high density of states near the dissociation limit, we perform photoassociation to form deeply bound excited molecules in a long-wavelength optical dipole trap. To enhance the free-bound Franck-Condon overlap, photoassociation is performed in the vicinity of the atom-molecule Feshbach resonance by ramping the magnetic field to about 49 G, which is slightly above the resonance located at about 48.2 G.
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Figure 2
The decay of the ${}^{23}{Na} {}^{40}K$ molecule number in the atom-molecule mixture for different photoassociation laser frequencies. The time evolution of the ${}^{23}{Na} {}^{40}K$ molecule number is recorded for different photoassociation laser frequency. The 1558 nm trapping laser is used as the photoassociation laser, the polarization of which is set to $σ^{+}$ . The solid lines are exponential fits to the data with $1 / e$ lifetimes of 13.6(5) ms at 192.358 THz, 9.3(6) ms at 192.357 THz, and 4.7(3) ms at 192.356 THz. Each point represents the average of 3–5 measurements, and error bars represent the standard error of the mean.
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Figure 3
The remaining number of ${}^{23}{Na} {}^{40}K$ molecules is plotted as a function of the frequency of the photoassociation laser. Each loss feature is dominantly excited by either $σ^{+}$ or $σ^{-}$ polarization. For comparison, the data points for both the polarizations and the results for pure molecules (p.m.) are shown. The solid lines are Gaussian fits to the loss features. The dashed lines are guides to the eye. Each point represents the average of 3–5 measurements, and error bars represent the standard error of the mean.
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Figure 4
The energy of the equilibrium geometries of the doublet states of ${}^{23}{Na} {}^{40}{K_{2}}$ molecules. (a) The four lowest equilibrium geometries above the $NaK (X {}^{1}{Σ^{+}}) + K (4 {}^{2}S)$ dissociation limit obtained by ab initio calculations. The energy of the atom-diatomic molecule dissociation limit is also shown. The red dashed line represents the spectral region studied in the current experiment. (b) The cumulative number of vibrational states as a function of their energy for the $1 {}^{2}{B_{2}}$ and $3 {}^{2}{A^{'}}$ states. The density of states is given by the gradient. The spectral regions in the current experiment are marked by circles.
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