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Science

Creation of an ultracold gas of triatomic molecules from an atom–diatomic molecule mixture

H. X. Yang, Jin Cao, Zhen Su, Jun Rui, Bo Zhao, Jian-Wei Pan

Published Dec 1, 2022• 39 citations
Publisher

Abstract

In recent years, there has been notable progress in the preparation and control of ultracold gases of diatomic molecules. The next experimental challenge is the production of ultracold polyatomic molecular gases. Here, we report the creation of an ultracold gas of 23 Na 40 K 2 triatomic molecules from a mixture of ground-state sodium-23–potassium-40 ( 23 Na 40 K) molecules and potassium-40 ( 40 K) atoms. The triatomic molecules were created by adiabatic magneto-association through an atom–diatomic molecule Feshbach resonance. We obtained clear evidence for the creation of triatomic molecules by directly detecting them using radio-frequency dissociation. Approximately 4000 triatomic molecules with a high-peak phase-space density of 0.05 could be created. The ultracold triatomic molecules can serve as a launchpad to probe the three-body potential energy surface and may be used to prepare quantum degenerate triatomic molecular gases.

DOI
10.1126/science.ade6307

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In recent years, there has been notable progress in the preparation and control of ultracold gases of diatomic molecules. The next experimental challenge is the production of ultracold polyatomic molecular gases. Here, we report the creation of an ultracold gas of 23 Na 40 K 2 triatomic molecules from a mixture of ground-state sodium-23–potassium-40 ( 23 Na 40 K) molecules and potassium-40 ( 40 K) atoms. The triatomic molecules were created by adiabatic magneto-association through an atom–diatomic molecule Feshbach resonance. We obtained clear evidence for the creation of triatomic molecules by directly detecting them using radio-frequency dissociation. Approximately 4000 triatomic molecules with a high-peak phase-space density of 0.05 could be created. The ultracold triatomic molecules can serve as a launchpad to probe the three-body potential energy surface and may be used to prepare quantum degenerate triatomic molecular gases.
We report on the preparation of a quantum degenerate mixture of 23Na40K^{23}\mathrm{Na}\phantom{\rule{0.16em}{0ex}}^{40}\mathrm{K}23Na40K molecules and 40K^{40}\mathrm{K}40K atoms. A deeply degenerate atomic mixture of 23Na^{23}\mathrm{Na}23Na and 40K^{40}\mathrm{K}40K atoms with a large number ratio (${N}_{F}/{N}_{B}\ensuremath{\approx}6$) is prepared in a large-volume optical dipole trap. About $3.0\ifmmode\times\else\texttimes\fi{}{10}^{4}\phantom{\rule{0.16em}{0ex}}^{23}\mathrm{Na}\phantom{\rule{0.16em}{0ex}}^{40}\mathrm{K}$ ground-state molecules are created through magnetoassociation followed by stimulated Raman adiabatic passage. The pure molecular gas is in the moderately degenerate regime. In the atom-molecule mixture, the elastic collisions provide a thermalization mechanism for the molecules. The calculated thermalization time constant is smaller than the lifetime of the molecules in the atom-molecule mixture. This indicates that the molecules may reach thermal equilibrium due to elastic atom-molecule collisions before notable losses take place. In a few tens of milliseconds, the degeneracy of the molecules is maintained. This time interval is long enough for the study of strongly interacting atom-molecule mixtures and the production of ultracold triatomic molecular gases.
Ultracold Feshbach molecules are a crucial intermediate step for the creation of quantum degenerate gases of strongly dipolar molecules. After coherent transfer to the rovibrational ground state, these dimers can realize stable dipolar gases with strong, tunable long-range interactions. Here, we report the creation of bosonic Na 23 K 41 Feshbach molecules by radio-frequency (RF) association. An RF pulse applied on the molecular side of an interspecies Feshbach resonance at 73.6(1) G associates up to 1.1 ( 1 ) × 10 4 molecules from a thermal mixture of Na 23 and K 41 atoms. Measurements of the binding energy reveal a broad resonance width of 5.1(2) G, facilitating robust control over interspecies interactions. The molecule lifetime in the presence of background atoms exceeds 2 ms and increases to approximately 7 ms after removal of Na 23 . These results constitute a key step toward the production of ultracold Na 23 K 41 ground-state molecules and for pursuing shielded evaporative cooling in the dipolar regime.
We report on the creation of ultracold gases of bosonic Feshbach molecules of NaCs.The molecules are associated from overlapping gases of Na and Cs using a Feshbach resonance at 864.12(5) G.We characterize the Feshbach resonance using bound-state spectroscopy, in conjunction with a coupled-channel calculation.By varying the temperature and atom numbers of the initial atomic mixtures, we demonstrate the association of NaCs gases over a wide dynamic range of molecule numbers and temperatures, reaching 70 nK for our coldest systems and a phase-space density near 0.1.This is an important stepping stone for the creation of degenerate gases of strongly dipolar NaCs molecules in their absolute ground state.
An apparatus for ultracold Na-40/41^{40/41}40/41K mixtures toward ground-state molecule production
We present an apparatus for producing ultracold Na- 40^{40}40 K and Na- 41^{41}41 K mixtures as a starting point for the formation of fermionic and bosonic ground-state NaK molecules. Sodium and potassium atoms are first loaded into a magneto-optical trap and transferred to an optically plugged magnetic trap, where the mixture undergoes sympathetic cooling via radio-frequency evaporation of Na. A key feature of the apparatus is its ability to switch between loading fermionic 40^{40}40 K and bosonic 41^{41}41 K by toggling three acousto-optic modulators. After further sympathetic cooling in an optical dipole trap, we obtain quantum-degenerate Bose–Fermi Na- 40^{40}40 K and Bose–Bose Na- 41^{41}41 K mixtures. This apparatus provides a robust starting point for creating ultracold Bose and Fermi gases of ground-state NaK molecules, opening pathways to explore strongly interacting dipolar quantum matter.
Ultracold bialkali polar molecules play a leading part at the frontline of quantum physics. They recently attract a lot of attention in the field of ultracold quantum chemistry, quantum many-body physics and quantum simulations. The key for their success is the rich internal level structure with rotational and vibrational degrees of freedom and their large electric dipole moments. Still, only a handful of molecular species are available at ultracold temperatures until now, although it is highly desirable to produce new molecular species to further expand the range of applications. Besides direct laser cooling methods for molecules, the assembly of heteronuclear ground-state molecules from ultracold atomic mixtures is the most promising approach for the creation of polar molecules. It includes the formation of weakly bound Feshbach molecules from the diatomic mixture and the subsequent two-photon stimulated Raman adiabatic passage (STIRAP) transfer to the rovibrational ground state. This creation strategy has been successfully demonstrated for the first time in the pioneering experiments at JILA with ultracold 40K87Rb molecules. Since then, only a few more molecular species from different alkali atoms have been created, namely 6Li23Na, 23Na40K, 23Na87Rb and 87Rb133Cs. In this thesis, I report the successful creation of a new species of ultracold polar ground-state molecules: 23Na39K. Starting from an ultracold mixture of bosonic 23Na and 39K atoms, weakly bound molecules are created. For this purpose, a Feshbach resonance in a high angular momentum scattering channel is chosen, experimentally identified and characterized. Close to this resonance the weakly bound Feshbach molecules are formed using resonant radio frequency radiation. For the two-photon ground-state transfer, a unique, highly specialized two-color laser system is designed and realized. It is used for one- and two-photon spectroscopy to identify the relevant transitions for the ground-state transfer. Based on the obtained data, a local model of the singlet-triplet mixed excited state manifolds is developed, with which the hyperfine structure and the magnetic field dependence is predicted with high accuracy. According to these findings, a suitable pathway to a single hyperfine ground state is chosen considering selection rules and experimental conditions such as laser polarization and beam alignment. To precisely determine the two-photon resonance condition for STIRAP, electromagnetically induced transparency measurements are performed. The ground-state transfer is then performed using STIRAP. The experimental findings regarding the STIRAP are successfully supported theoretically by a model based on a five-level master equation. The pure molecular gas shows evidence for two-body dominated loss mechanisms, such as sticky four-body collisions. The molecule-atom mixture of 23Na39K+39K reveals an unexpectedly low loss rate coefficient although sticky three-body collisions are assumed to occur. This behavior demands further investigations regarding different hyperfine collisional chan-nels and magnetic field regions.
A theoretical model is proposed for the formation of ultracold ground-state triatomic molecules in weakly bound energy levels. The process is driven by the electric component of a microwave field, which induces the association of an ultracold atom colliding with an ultracold diatomic molecule. This model is exemplified using K 39 atoms and Na 23 K 39 molecules, both in their ground states, a scenario of experimental relevance. The model assumes that the dynamics of the association are dominated by the long-range van der Waals interaction between K 39 and Na 23 K 39 . The electric microwave association mechanism relies on the intrinsic electric dipole moment of Na 23 K 39 , which drives transitions between its lowest rotational levels ( j =0 and j =1). The energies of the uppermost triatomic energy levels are computed by numerically solving coupled Schrödinger equations using the Mapped Fourier Grid Hamiltonian method. Measurable association rates are derived within the framework of a perturbative approach. This method of electric microwave association provides an alternative to atom-molecule association via magnetic Feshbach resonances for forming ultracold, deeply bound triatomic molecules, and is applicable to a wide range of polar diatomic molecules.