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arXiv:0904.0935 · Laser Physics

Collisions of ultracold trapped cesium Feshbach molecules

Francesca Ferlaino, Steven Knoop, M. Berninger, M. Mark, Hanns‐Christoph Nägerl, Rudolf Grimm

Published Jul 29, 2009• 13 citations
arXivHTMLPDF

Abstract

We study collisions in an optically trapped, pure sample of ultracold Cs 2 molecules in various internal states. The molecular gas is created by Feshbach association from a near-degenerate atomic gas, with adjustable temperatures in the nanokelvin range. We identify several narrow loss resonances, which point to the coupling to more complex molecular states and may be interpreted as Feshbach resonances in dimerdimer interactions. Moreover, in some molecular states we observe a surprising temperature dependence in collisional loss. This shows that the situation cannot be understood in terms of the usual simple threshold behavior for inelastic two-body collisions. We interpret this observation as further evidence for a more complex molecular structure beyond the well-understood dimer physics.

DOI
10.1134/s1054660x0917006x

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•Jan 1, 2009•0 citations
We study collisions in an optically trapped, pure sample of ultracold Cs 2 molecules in various internal states. The molecular gas is created by Feshbach association from a near-degenerate atomic gas, with adjustable temperatures in the nanokelvin range. We identify several narrow loss resonances, which point to the coupling to more complex molecular states and may be interpreted as Feshbach resonances in dimerdimer interactions. Moreover, in some molecular states we observe a surprising temperature dependence in collisional loss. This shows that the situation cannot be understood in terms of the usual simple threshold behavior for inelastic two-body collisions. We interpret this observation as further evidence for a more complex molecular structure beyond the well-understood dimer physics.
We study inelastic collisions in a pure, trapped sample of Feshbach molecules made of bosonic cesium atoms in the quantum halo regime. We measure the relaxation rate coefficient for decay to lower-lying molecular states and study the dependence on scattering length and temperature. We identify a pronounced loss minimum with varying scattering length along with a further suppression of loss with decreasing temperature. Our observations provide insight into the physics of a few-body quantum system that consists of four identical bosons at large values of the two-body scattering length.
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.
Ultracold quantum gases are ideal to explore few-body physics because of the tunable two-body interaction. The focus of our research is the exploration of quantum mechanical three-body and four-body systems. For this we investigate inelastic collisions between ultracold Cs atoms and Cs2 molecules. The preparation of the cold atomic sample takes place in an optical dipole trap. We associate the molecules from an ultracold atomic sample by magnetically tunable Feshbach resonances. In this work I report on the implementation of a new dipole trap. The new trap allows us to tune the position and the shape of the dipole potential during the running experiment and to prepare atomic and molecular samples at low temperatures in a convenient way. This new trap opens up a way to many exciting novel experiments. In this thesis I describe the experimental setup focusing on the new system. I also discuss the creation and preparation of Cs2 dimers by Feshbach association from an ultracold atomic sample. The obtained ultracold atomic and molecular sample serves as a starting point to study inelastic atom-dimer and dimer-dimer collisions. Here I will discuss a measurement in which the atom-dimer collision loss coefficient is determined for different external magnetic fields. Future experimental directions will be the research of three-body and four-body physics at high magnetic fields, in particular the investigation of Efimov physics in the ultracold regime.
We observe magnetically tuned collision resonances for ultracold Cs2 molecules stored in a CO2-laser trap. By magnetically levitating the molecules against gravity, we precisely measure their magnetic moment. We find an avoided level crossing which allows us to transfer the molecules into another state. In the new state, two Feshbach-like collision resonances show up as strong inelastic loss features. We interpret these resonances as being induced by Cs4 bound states near the molecular scattering continuum. The tunability of the interactions between molecules opens up novel applications such as controlled chemical reactions and synthesis of ultracold complex molecules.
We report the observation of a dimer-dimer inelastic collision resonance for ultracold Feshbach molecules made of bosonic sodium and rubidium atoms. This resonance, which we attribute to the crossing of the dimer-dimer threshold with a heteronuclear tetramer state, manifests itself as a pronounced inelastic loss peak of dimers when the interspecies scattering length between the constituent atoms is tuned. Near this resonance, a strong modification of the temperature dependence of the dimer-dimer scattering is observed. Our result provides insight into the heteronuclear four-body system consisting of heavy and light bosons and offers the possibility of investigating ultracold molecules with tunable interactions.
We investigate quantum-mechanical interactions between ultracold rubidium and cesium in an optical trap at temperatures of a few microkelvin. Our results provide, on the one hand, an experimental key to understand the collisional properties and, on the other hand, a tool to control the interspecies interactions. By performing loss measurements we locate several Feshbach resonances, which provide insight into the energy structure of weakly bound RbCs molecules near the dissociation threshold and allow for the production of such heteronuclear Feshbach molecules. In the future we will transfer these loosely-bound molecules into the absolute internal ground state. The availability of ultracold heteronuclear ground state molecules will open the door to investigate phenomena associated with ultracold polar quantum gases. In our new experimental set-up we are able to trap and cool rubidium and cesium atoms in their lowest internal states. First we load both species into a two-color magneto-optical trap, having full control over the single-species atom number. We extend the technique of degenerate Raman-sideband cooling to a two-color version, which is able to simultaneously cool and polarize both rubidium and cesium. Thereafter we load the atoms into a levitated crossed optical dipole trap. Because of the presence of the gradient magnetic field the trap is highly state selective and consequently provides perfect spin-polarization of the sample. Furthermore, a coincidence of the magnetic-moment-to-mass ratios of the two species allows for simultaneous levitation of both, which assures an almost perfect spatial overlap between the species. We perform Feshbach spectroscopy in two different spin channels of the mixture within a magnetic field ranging from 20 to 300G. In the lowest spin combination of the species we locate 23 interspecies Feshbach resonances, while in a higher spin mixture we find 2 resonances. The high number of resonances found within this range of magnetic field is unusual for alkali mixtures. The presence of many resonances points to scattering properties of the mixture, which include higher-order coupling mechanisms. The obtained data on the Feshbach spectroscopy provide, on one hand, fundamental experimental input to characterize the Rb-Cs scattering properties and, on the other hand, identification of possible starting points for the association of ultracold heteronuclear RbCs molecules. In addition we show preliminary results on spectroscopy of the binding energy of RbCs dimers, based on a modulation of the magnetic field. The recently obtained Bose-Einstein condensate of Cs atoms, which represents a benchmark for the performance of the present apparatus, will be discussed as well as the potential pathways towards a double-degenerate mixture. We follow two main goals, first the production of a double Bose-Einstein condensate and, second, the transfer of shallow bound Feshbach molecules to the absolute internal ground state. The production of heteronuclear ground state molecules allows to enter the world of ultracold polar quantum systems.