The collision of molecules at ultracold temperatures is of great importance to understand the chemical interactions at the quantum regime. Although much theoretical work has been devoted to this, experimental data are only sparsely available, mainly because of the difficulty in producing ground-state molecules at ultracold temperatures. We report here the creation of optically trapped samples of ground-state bosonic sodium-rubidium molecules with precisely controlled internal states and, enabled by this, a detailed study on the inelastic loss with and without the NaRb + NaRb → Na2 + Rb2 chemical reaction. Contrary to intuitive expectations, we observed very similar loss and heating, regardless of the chemical reactivities. In addition, as evidenced by the reducing loss rate constants with increasing temperatures, we found that these collisions are already outside the Wigner region although the sample temperatures are sub-microkelvin. Our measurement agrees semiquantitatively with models based on long-range interactions but calls for a deeper understanding on the short-range physics for a more complete interpretation.

https://advances.sciencemag.org/content/advances/4/1/eaaq0083.full.pdf

Collisions of ultracold 23Na87Rb molecules with controlled chemical reactivities

Understanding and controlling collisions is crucial to the burgeoning field of ultracold molecules. All experiments so far have observed fast loss of molecules from the trap. However, the dominant mechanism for collisional loss is not well understood when there are no allowed 2-body loss processes. Here we experimentally investigate collisional losses of nonreactive ultracold 87Rb133Cs molecules, and compare our findings with the sticky collision hypothesis that pairs of molecules form long-lived collision complexes. We demonstrate that loss of molecules occupying their rotational and hyperfine ground state is best described by second-order rate equations, consistent with the expectation for complex-mediated collisions, but that the rate is lower than the limit of universal loss. The loss is insensitive to magnetic field but increases for excited rotational states. We demonstrate that dipolar effects lead to significantly faster loss for an incoherent mixture of rotational states.

/pdf/sticky-collisions-of-ultracold-rbcs-molecules-40y8vbyl8b.pdf

Sticky collisions of ultracold RbCs molecules.

The collision of molecules at ultracold temperatures is of great importance for understanding the chemical interactions at the quantum regime While much theoretical work has been devoted to this, experimental data are only available sparsely mainly due to the difficulty in producing ground-state molecules at ultracold temperatures We report here the creation of optically trapped samples of ground-state bosonic sodium-rubidium molecules with precisely controlled internal states and, enabled by this, a detailed study on the inelastic loss with and without the NaRb + NaRb $\rightarrow$ Na$_2$ + Rb$_2$ chemical reaction Contrary to intuitive expectations, we observed very similar loss and heating, regardless of the chemical reactivities In addition, as evidenced by the reducing loss rate constants with increasing temperatures, we found that these collisions are already outside the Wigner region even though the sample temperatures are sub-microkelvin Our measurement agrees semi-quantitatively with models based on long-range interactions, but calls for a deeper understanding on the short-range physics for a more complete interpretation

Collisions of ultracold ^{23}Na^{87}Rb molecules with controlled chemical reactivities

We show that the lifetime of ultracold ground-state ^{87}Rb^{133}Cs molecules in an optical trap is limited by fast optical excitation of long-lived two-body collision complexes. We partially suppress this loss mechanism by applying square-wave modulation to the trap intensity, such that the molecules spend 75% of each modulation cycle in the dark. By varying the modulation frequency, we show that the lifetime of the collision complex is 0.53±0.06  ms in the dark. We find that the rate of optical excitation of the collision complex is 3_{-2}^{+4}×10^{3}  W^{-1} cm^{2} s^{-1} for λ=1550  nm, leading to a lifetime of <100  ns for typical trap intensities. These results explain the two-body loss observed in experiments on nonreactive bialkali molecules.

Loss of Ultracold ^{87}Rb^{133}Cs Molecules via Optical Excitation of Long-Lived Two-Body Collision Complexes.

By applying a circularly polarized and slightly blue-detuned microwave field with respect to the first excited rotational state of a dipolar molecule, one can engineer a long-range, shallow potential well in the entrance channel of the two colliding partners. As the applied microwave ac field is increased, the long-range well becomes deeper and can support a certain number of bound states, which in turn bring the value of the molecule-molecule scattering length from a large negative value to a large positive one. We adopt an adimensional approach where the molecules are described by a rescaled rotational constant B[over ˜]=B/s_{E_{3}} where s_{E_{3}} is a characteristic dipolar energy. We found that molecules with B[over ˜]>10^{8} are immune to any quenching losses when a sufficient ac field is applied, the ratio elastic to quenching processes can reach values above 10^{3}, and that the value and sign of the scattering length can be tuned. The ability to control the molecular scattering length opens the door for a rich, strongly correlated, many-body physics for ultracold molecules, similar to that for ultracold atoms.

https://hal.archives-ouvertes.fr/hal-02078386/document

Controlling the Scattering Length of Ultracold Dipolar Molecules.

A systematic study of the leading isotropic van der Waals coefficients for the alkali-metal atom+molecule and molecule+molecule systems is presented. Dipole moments and static and dynamic dipole polarizabilities are calculated employing high-level quantum chemistry calculations. The dispersion, induction, and rotational parts of the isotropic van der Waals coefficient are evaluated. The known van der Waals coefficients are then used to derive characteristics essential for simple models of the collisions involving the corresponding ultracold polar molecules.

M. Kodrycka

Papers

van der Waals coefficients for systems with ultracold polar alkali-metal molecules