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arXiv:cond-mat/0411258 · Physical Review Letters

Observation of Feshbach-Like Resonances in Collisions between Ultracold Molecules

Cheng Chin, Tobias Kraemer, Manfred J. Mark, Jens Herbig, P. Waldburger, Hanns‐Christoph Nägerl, Rudolf Grimm

Published Apr 1, 2005• 153 citations
arXivHTMLPDF

Abstract

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.

DOI
10.1103/physrevlett.94.123201

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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 explore the rich internal structure of Cs2{\text{Cs}}_{2}Cs2​ Feshbach molecules. Pure ultracold molecular samples are prepared in a CO2{\text{CO}}_{2}CO2​-laser trap, and a multitude of weakly bound states is populated by elaborate magnetic-field ramping techniques. Our methods use different Feshbach resonances as input ports and various internal level crossings for controlled state transfer. We populate higher partial-wave states of up to eight units of rotational angular momentum (lll-wave states). We investigate the molecular structure by measurements of the magnetic moments for various states. Avoided level crossings between different molecular states are characterized through the changes in magnetic moment and by a Landau-Zener tunneling method. Based on microwave spectroscopy, we present a precise measurement of the magnetic-field-dependent binding energy of the weakly bound sss-wave state that is responsible for the large background scattering length of Cs. This state is of particular interest because of its quantum-halo character.
We have observed and located more than 60 magnetic field-induced Feshbach resonances in ultracold collisions of ground-state 133Cs^{133}\mathrm{Cs}133Cs atoms. Multiple extremely weak Feshbach resonances associated with ggg-wave molecular states are detected through variations in the radiative collision cross sections. The Feshbach spectroscopy allows us to determine the interactions between ultracold cesium atoms and the molecular energy structure near the dissociation continuum with unprecedented precision. Our work not only represents a very successful collaboration of experimental and theoretical efforts, but also provides essential information for cesium Bose-Einstein condensation, Cs2{\mathrm{Cs}}_{2}Cs2​ molecules, and atomic clock experiments.
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.
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 have carried out bound-state and low-energy quantum scattering calculations on He + NH (triplet Sigma) in magnetic fields, with the NH molecule in its n=1 rotationally excited states. We have explored the pattern of levels as a function of magnetic field and identified the nearly good quantum numbers in different regimes. We have used the bound-state calculations to locate low-energy Feshbach resonances. When the magnetic field is used to tune across such a resonance, the real and imaginary part of the scattering length show asymmetric oscillations and peaks with amplitude between 1 and 3 Angstroms. The scattering length does not pass through a pole at resonance. The resonant behavior is characterized by a complex resonant scattering length a_res. The corresponding inelastic cross sections show troughs as well as peaks near resonance. This may be important for efforts to achieve evaporative and sympathetic cooling for molecules, because it offers the hope that inelastic trap losses can be reduced by tuning close to a Feshbach resonance.
In this Letter, we observe a novel resonant mechanism, namely, the modulation-induced Feshbach resonance. By applying a far-detuned laser to the cesium D_{2} transition with intensity modulation, we periodically modulate the energy levels of atomic collisional states. This periodic modulation connects the free-scattering states to shallow molecular states. At specific frequencies, significant atom loss is observed, which corresponds to the resonant coupling between these two types of states. This precisely corresponds to a form of Feshbach resonance, yet in the frequency domain rather than the magnetic-field domain. Using this method, we can directly scan the energy spectrum of molecular bound states without synthesizing any molecules. In addition to these bound states, we can also probe the molecular states embedded in the continuum, which are typically very difficult to detect by the conventional methods based on molecular synthesis. Moreover, by using a far-detuned laser instead of a magnetic field, it enables spatially dependent control over atomic interactions, coupling multiple levels simultaneously and inducing new Feshbach resonances for those atoms that do not have conventional magnetic resonances. Therefore, we believe that this new resonant mechanism offers new opportunities for controlling atomic and molecular interactions in quantum simulations.