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arXiv:0710.4052 · Physical Review Letters

Metastable Feshbach Molecules in High Rotational States

Steven Knoop, Manfred J. Mark, Francesca Ferlaino, Johann Georg Danzl, Tobias Kraemer, Hanns‐Christoph Nägerl, Rudolf Grimm

Published Feb 29, 2008• 26 citations
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Abstract

We experimentally demonstrate Cs2 Feshbach molecules well above the dissociation threshold, which are stable against spontaneous decay on the time scale of 1 s. An optically trapped sample of ultracold dimers is prepared in a high rotational state and magnetically tuned into a region with a negative binding energy. The metastable character of these molecules arises from the large centrifugal barrier in combination with negligible coupling to states with low rotational angular momentum. A sharp onset of dissociation with increasing magnetic field is mediated by a crossing with a lower rotational dimer state and facilitates dissociation on demand with a well-defined energy.

DOI
10.1103/physrevlett.100.083002

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We experimentally demonstrate Cs2 Feshbach molecules well above the dissociation threshold, which are stable against spontaneous decay on the time scale of 1 s. An optically trapped sample of ultracold dimers is prepared in a high rotational state and magnetically tuned into a region with a negative binding energy. The metastable character of these molecules arises from the large centrifugal barrier in combination with negligible coupling to states with low rotational angular momentum. A sharp onset of dissociation with increasing magnetic field is mediated by a crossing with a lower rotational dimer state and facilitates dissociation on demand with a well-defined energy.
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 investigated magnetically tunable Feshbach resonances in ultracold collisions of Rb with Yb in its metastable 3P2{}^{3}{P}_{2}3P2​ and 3P0{}^{3}{P}_{0}3P0​ states, using coupled-channel scattering and bound-state calculations. For the 3P2{}^{3}{P}_{2}3P2​ state, we find sharp resonances when both atoms are in their lowest Zeeman sublevels. However, these resonances are decayed by inelastic processes that produce Yb atoms in 3P1{}^{3}{P}_{1}3P1​ and 3P0{}^{3}{P}_{0}3P0​ states. The molecules that might be produced by magnetoassociation at the 3P2{}^{3}{P}_{2}3P2​ thresholds can decay by similar pathways and would have lifetimes no more than a few microseconds. For the 3P0{}^{3}{P}_{0}3P0​ state, by contrast, there are resonances that are promising for magnetoassociation. There are resonances due to both rotating and nonrotating molecular states that are significantly stronger than the analogous resonances for Yb(1S\mathrm{Yb}{(}^{1}SYb(1S). The ones due to rotating states are denser in magnetic field; in contrast to Yb(1S\mathrm{Yb}{(}^{1}SYb(1S), they exist even for bosonic isotopes of Yb(3P0\mathrm{Yb}{(}^{3}{P}_{0}Yb(3P0​).
We report on the observation of an elementary exchange process in an optically trapped ultracold sample of atoms and Feshbach molecules. We can magnetically control the energetic nature of the process and tune it from endoergic to exoergic, enabling the observation of a pronounced threshold behavior. In contrast to relaxation to more deeply bound molecular states, the exchange process does not lead to trap loss. We find excellent agreement between our experimental observations and calculations based on the solutions of three-body Schrödinger equation in the adiabatic hyperspherical representation. The high efficiency of the exchange process is explained by the halo character of both the initial and final molecular states.
The dissociation of ultracold molecules was studied by ramping an external magnetic field through a Feshbach resonance. The observed dissociation energies directly yielded the strength of the atom-molecule coupling. They showed nonlinear dependence on the ramp speed. This was explained by a Wigner threshold law which predicts that the decay rate of the molecules above threshold increases with the density of states. In addition, inelastic molecule-molecule and molecule-atom collisions were characterized.
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.
The spontaneous dissociation of 85Rb dimers in the highest lying vibrational level has been observed in the vicinity of the Feshbach resonance that was used to produce them. The molecular lifetime shows a strong dependence on magnetic field, varying by 3 orders of magnitude between 155.5 G and 162.2 G. Our measurements are in good agreement with theoretical predictions in which molecular dissociation is driven by inelastic spin relaxation. Molecule lifetimes of tens of milliseconds can be achieved within approximately a 1 G wide region directly above the Feshbach resonance.