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arXiv:2109.02602 · Physical Review A

Electric field dependence of complex-dominated ultracold molecular collisions

Goulven Quéméner, James F. E. Croft, John L. Bohn

Published Jan 13, 2022• 10 citations
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Abstract

Recent experiments on ultracold nonreactive dipolar molecules have observed high two-body losses, even though these molecules can undergo neither inelastic, nor reactive (as they are in their absolute ground state), nor light-assisted collisions (if they are measured in the dark). In the presence of an electric field these losses seem to be near universal (the probability of loss at short range is near unity) while in the absence of it the losses seem nonuniversal. To explain these observations we propose a simple model based on the mixing effect of an electric field on the states of the two diatomic molecules at long range and on the density of states of the tetramer complex formed at short range, believed to be responsible for the losses. We apply our model to collisions of ground-state molecules of endothermic systems, of current experimental interest.

DOI
10.1103/physreva.105.013310

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Recent experiments on ultracold nonreactive dipolar molecules have observed high two-body losses, even though these molecules can undergo neither inelastic, nor reactive (as they are in their absolute ground state), nor light-assisted collisions (if they are measured in the dark). In the presence of an electric field these losses seem to be near universal (the probability of loss at short range is near unity) while in the absence of it the losses seem nonuniversal. To explain these observations we propose a simple model based on the mixing effect of an electric field on the states of the two diatomic molecules at long range and on the density of states of the tetramer complex formed at short range, believed to be responsible for the losses. We apply our model to collisions of ground-state molecules of endothermic systems, of current experimental interest.
Understanding collisions between ultracold molecules is crucial for making stable molecular quantum gases and harnessing their rich internal degrees of freedom for quantum engineering. Transient complexes can strongly influence collisional physics, but in the ultracold regime, key aspects of their behavior have remained unknown. To explain experimentally observed loss of ground-state molecules from optical dipole traps, it was recently proposed that molecular complexes can be lost due to photoexcitation. By trapping molecules in a repulsive box potential using laser light near a narrow molecular transition, we are able to test this hypothesis with light intensities three orders of magnitude lower than what is typical in red-detuned dipole traps. This allows us to investigate light-induced collisional loss in a gas of nonreactive fermionic 23Na40K^{23}\mathrm{Na}^{40}\mathrm{K}23Na40K molecules. Even for the lowest intensities available in our experiment, our results are consistent with universal loss, meaning unit loss probability inside the short-range interaction potential. Our findings disagree by at least two orders of magnitude with latest theoretical predictions, showing that crucial aspects of molecular collisions are not yet understood and provide a benchmark for the development of new theories.
Abstract 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 87 Rb 133 Cs 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.
Abstract 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.
The dipolar collision between ultracold polar molecules is an important topic both by its own right from the fundamental point of view and for the successful exploration of many-body physics with strong and longrange dipolar interactions.Here, we report the investigation of collisions between ultracold ground-state sodium-rubidium molecules in electric fields with induced electric dipole moments as large as 0.7 D. We observe a stepwise enhancement of losses due to the coupling between different partial waves induced by the increasingly stronger anisotropic dipolar interactions.Varying the temperature of our sample, we find good agreement with theoretical loss rates assuming complex formation as the main loss process.Our results shed new light on the understanding of complex molecular collisions in the presence of strong dipolar interactions and also demonstrate the versatility of modifying molecular interactions with electric fields.
Abstract Understanding ultracold collisions involving molecules is of fundamental importance for current experiments, where inelastic collisions typically limit the lifetime of molecular ensembles in optical traps. Here we present a broad study of optically trapped ultracold RbCs molecules in collisions with one another, in reactive collisions with Rb atoms, and in nonreactive collisions with Cs atoms. For experiments with RbCs alone, we show that by modulating the intensity of the optical trap, such that the molecules spend 75% of each modulation cycle in the dark, we partially suppress collisional loss of the molecules. This is evidence for optical excitation of molecule pairs mediated via sticky collisions. We find that the suppression is less effective for molecules not prepared in the spin-stretched hyperfine ground state. This may be due either to longer lifetimes for complexes in the dark or to laser-free decay pathways. For atom–molecule mixtures, RbCs + Rb and RbCs + Cs, we demonstrate that the rate of collisional loss of molecules scales linearly with the density of atoms. This indicates that, in both cases, the loss of molecules is rate-limited by two-body atom–molecule processes. For both mixtures, we measure loss rates that are below the thermally averaged universal limit.
The prospects for shielding ultracold, paramagnetic, dipolar molecules from inelastic and chemical collisions are investigated. Molecules placed in their first rotationally excited states are found to exhibit effective long-range repulsion for applied electric fields above a certain critical value, as previously shown for nonparamagnetic molecules. This repulsion can safely allow the molecules to scatter while reducing the risk of inelastic or chemically reactive collisions. Several molecular species of $^{2}\mathrm{\ensuremath{\Sigma}}$ molecules of experimental interest---RbSr, SrF, BaF, and YO---are considered, and all are shown to exhibit orders of magnitude suppression in quenching rates in a sufficiently strong laboratory electric field. It is further shown that, for these molecules described by Hund's coupling case (b), electronic and nuclear spins play the role of spectator with respect to the shielding.