Rovibrational cooling of molecules by optical pumping

Quantum control of reactions and collisions at ultralow temperatures

At temperatures close to absolute zero, the molecular reactions and collisions are dominantly governed by quantum mechanics. Remarkable quantum phenomena such as quantum tunneling, quantum threshold behavior, quantum resonances, quantum interference, and quantum statistics are expected to be the main features in ultracold reactions and collisions. Ultracold molecules offer great opportunities and challenges in the study of these intriguing quantum phenomena in molecular processes. In this article, we review the recent progress in the preparation of ultracold molecules and the study of ultracold reactions and collisions using ultracold molecules. We focus on the controlled ultracold chemistry and the scattering resonances at ultralow temperatures. The challenges in understanding the complex ultracold reactions and collisions are also discussed.

On the Feasibility of Rovibrational Laser Cooling of Radioactive RaF+ and RaH+ Cations

Polar radioactive molecules have been suggested to be exceptionally sensitive systems in the search for signatures of symmetry-violating effects in their structure. Radium monofluoride (RaF) possesses an especially attractive electronic structure for such searches, as the diagonality of its Franck-Condon matrix enables the implementation of direct laser cooling for precision experiments. To maximize the sensitivity of experiments with short-lived RaF isotopologues, the molecular beam needs to be cooled to the rovibrational ground state. Due to the high kinetic energies and internal temperature of extracted beams at radioactive ion beam (RIB) facilities, in-flight rovibrational cooling would be restricted by a limited interaction timescale. Instead, cooling techniques implemented on ions trapped within a radiofrequency quadrupole cooler-buncher can be highly efficient due to the much longer interaction times (up to seconds). In this work, the feasibility of rovibrationally cooling trapped RaF+ and RaH+ cations with repeated laser excitation is investigated. Due to the highly diagonal nature between the ionic ground state and states in the neutral system, any reduction of the internal temperature of the molecular ions would largely persist through charge-exchange without requiring the use of cryogenic buffer gas cooling. Quasirelativistic X2C and scalar-relativistic ECP calculations were performed to calculate the transition energies to excited electronic states and to study the nature of chemical bonding for both RaF+ and RaH+. The results indicate that optical manipulation of the rovibrational distribution of trapped RaF+ and RaH+ is unfeasible due to the high electronic transition energies, which lie beyond the capabilities of modern laser technology. However, more detailed calculations of the structure of RaH+ might reveal possible laser-cooling pathways.

Vibrational energies of some diatomic molecules for a modified and deformed potential

A molecular potential model is proposed and the solutions of the radial Schrӧdinger equation in the presence of the proposed potential is obtained. The energy equation and its corresponding radial wave function are calculated using the powerful parametric Nikiforov–Uvarov method. The energies of cesium dimer for different quantum states were numerically obtained for both negative and positive values of the deformed and adjustable parameters. The results for sodium dimer and lithium dimer were calculated numerically using their respective spectroscopic parameters. The calculated values for the three molecules are in excellent agreement with the observed values. Finally, we calculated different expectation values and examined the effects of the deformed and adjustable parameters on the expectation values.

Probing Fundamental Symmetries of Deformed Nuclei in Symmetric Top Molecules

Precision measurements of Schiff moments in heavy, deformed nuclei are sensitive probes of beyond standard model T, P violation in the hadronic sector. While the most stringent limits on Schiff moments to date are set with diamagnetic atoms, polar polyatomic molecules can offer higher sensitivities with unique experimental advantages. In particular, symmetric top molecular ions possess K doublets of opposite parity with especially small splittings, leading to full polarization at low fields, internal comagnetometer states useful for rejection of systematic effects, and the ability to perform sensitive searches for T, P violation using a small number of trapped ions containing heavy exotic nuclei. We consider the symmetric top cation ^{225}RaOCH_{3}^{+} as a prototypical and candidate platform for performing sensitive nuclear Schiff measurements and characterize in detail its internal structure using relativistic ab initio methods. The combination of enhancements from a deformed nucleus, large polarizability, and unique molecular structure make this molecule a promising platform to search for fundamental symmetry violation even with a single trapped ion.

Continuous Loading of Ultracold Ground-State ^{85}Rb_{2} Molecules in a Dipole Trap Using a Single Light Beam

We have developed an approach to continuously load ultracold ^{85}Rb_{2} vibrational ground-state molecules into a crossed optical dipole trap from a magneto-optical trap. The technique relies on a single high-power light beam with a broad spectrum superimposed onto a narrow peak at an energy of about 9400  cm^{-1}. This single laser source performs all the required steps: the short-range photoassociation creating ground-state molecules after radiative emission, the cooling of the molecular vibrational population down to the lowest vibrational level v_{X}=0, and the optical trapping of these molecules. Furthermore, we probe by depletion spectroscopy and determine that 75% of the v_{X}=0 ground-state molecules are in the three lowest rotational levels J_{X}=0, 1, 2. The lifetime of the ultracold molecules in the optical dipole trap is limited to about 70 ms by off-resonant light scattering. The proposed technique opens perspectives for the formation of new molecular species in the ultracold domain, which are not yet accessible by well-established approaches.

A dynamical process of optically trapped singlet ground state 85Rb133Cs molecules produced via short-range photoassociation

We investigate the dynamical process of optically trapped X¹Σ⁺ (v'' = 0) state ⁸⁵Rb¹³³Cs molecules distributed in J'' = 1 and J'' = 3 rotational states. The considered molecules, formed from short-range photoassociation of mixed cold atoms, are subsequently confined in a crossed optical dipole trap. Based on a phenomenological rate equation, we provide a detailed study of the dynamics of ⁸⁵Rb¹³³Cs molecules during the loading and holding processes. The inelastic collisions of ⁸⁵Rb¹³³Cs molecules in the X¹Σ⁺ (v'' = 0, J'' = 1 and J'' = 3) states with ultracold ⁸⁵Rb (or ¹³³Cs) atoms are measured to be 1.0 (2) × 10^-10 cm³ s^-1 (1.2 (3) × 10^-10 cm³ s^-1). Our work provides a simple and generic procedure for studying the dynamical process of trapped cold molecules in the singlet ground states.

Improved Radio-Frequency Magneto-Optical Trap of SrF Molecules

We report the production of ultracold, trapped strontium monofluoride (SrF) molecules with number density and phase-space density significantly higher than previously achieved. These improvements are enabled by three distinct changes to our recently-demonstrated scheme for radio-frequency magneto-optical trapping of SrF: modification of the slowing laser beam geometry, addition of an optical pumping laser, and incorporation of a compression stage to the magneto-optical trap. With these improvements, we observe a trapped sample of SrF molecules at density 2.5×10⁵  cm^-3 and phase-space density 6×10^-14 , each a factor of 4 greater than in previous work. Under different experimental conditions, we observe trapping of up to 10⁴ molecules, a factor of 5 greater than in previous work. Finally, by reducing the intensity of the applied trapping light, we observe molecular temperatures as low as 250 μK.

The determination of potential energy curve and dipole moment of the (5)0(+) electronic state of (85)Rb(133)Cs molecule by high resolution photoassociation spectroscopy

We present the formation of ultracold (85)Rb(133)Cs molecules in the (5)0(+) electronic state by photoassociation and their detection via resonance-enhanced two-photon ionization. Up to v = 47 vibrational levels including the lowest v = 0 vibrational and lowest J = 0 levels are identified with rotationally resolved high resolution photoassociation spectra. Precise Dunham coefficients are determined for the (5)0(+) state with high accuracy, then the Rydberg-Klein-Rees potential energy curve is derived. The electric dipole moments with respect to the vibrational numbers of the (5)0(+) electronic state of (85)Rb(133)Cs molecule are also measured in the range between 1.9 and 4.8 D. These comprehensive studies on previously unobserved rovibrational levels of the (5)0(+) state are helpful to understand the molecular structure and discover suitable transition pathways for transferring ultracold atoms to deeply bound rovibrational levels of the electronic ground state.

Rotational Cooling of Trapped Polyatomic Molecules

Controlling the internal degrees of freedom is a key challenge for applications of cold and ultracold molecules. Here, we demonstrate rotational-state cooling of trapped methyl fluoride molecules (CH_{3}F) by optically pumping the population of 16 M sublevels in the rotational states J=3, 4, 5 and 6 into a single level. By combining rotational-state cooling with motional cooling, we increase the relative number of molecules in the state J=4, K=3, M=4 from a few percent to over 70%, thereby generating a translationally cold (≈30  mK) and nearly pure state ensemble of about 10^{6} molecules. Our scheme is extendable to larger sets of initial states, other final states, and a variety of molecule species, thus paving the way for internal-state control of ever-larger molecules.

Quantum phases of quadrupolar Fermi gases in optical lattices

We introduce a new platform for quantum simulation of many-body systems based on nonspherical atoms or molecules with zero dipole moments but possessing a significant value of electric quadrupole moments. We consider a quadrupolar Fermi gas trapped in a 2D square optical lattice, and show that the peculiar symmetry and broad tunability of the quadrupole-quadrupole interaction results in a rich phase diagram encompassing unconventional BCS and charge density wave phases, and opens up a perspective to create a topological superfluid. Quadrupolar species, such as metastable alkaline-earth atoms and homonuclear molecules, are stable against chemical reactions and collapse and are readily available in experiment at high densities.