Metastable Feshbach Molecules
in High Rotational States

Institut für Experimentalphysik,
University of Innsbruck, and
Austrian Academy of Sciences,
Innsbruck, Austria

Highly rotating Feshbach molecules refuse to dissociate

We have demonstrated the metastable character of ultracold dimers in a high rotational state above the dissociation threshold. In our experiments, we observed Cs2 Feshbach molecules in an l-wave state to be stable against dissociative decay on a time scale of at least 1 s. The large centrifugal barrier suppresses tunneling to the l-wave scattering continuum while the coupling to lower partial waves is extremely small.

Feshbach molecules are characterized by their rotational quantum number. A strong coupling between the atomic and molecular state is usually only present for s-wave molecules, i.e. molecular states with rotational quantum number l=0, resulting in broad Feshbach resonances. For molecular states with nonzero rotational quantum numbers this coupling is much weaker, however in some case strong enough to be observed as Feshbach resonances and to produce these molecules. Especially for Cs this coupling is relatively strong and allows the formation of d-wave (l=2) and g-wave (l=4) Feshbach molecules [1,2]. Feshbach resonances caused by molecular states with l>4 have not been observed, indicating the negligible coupling between those molecular states and the atomic continuum.

But does this mean that one cannot produce Feshbach molecules with l>4 ? No ! Coupling between different molecular states allows state-to-state transfer. As long as the coupling between molecular states with different rotational quantum number is not too small, one can step by step increase the rotational quantum number. For instance, starting from a molecular sample in a g-wave state one can transfer the sample in a l-wave (l=8) state [2]. Now the interesting question rises: what happens with these molecules when we (magnetically) tune them above the dissociation threshold ?

Nothing ! We have experimentally shown that dissociative decay is suppressed and lifetimes on the order of 1 s are possible. Thus one can create a sample of metastable Feshbach molecules and enter the energy regime above the dissociation threshold. This opens up the possibility to create novel metastable quantum states with strong pair correlations.

Metastable Feshbach Molecules in High Rotational States
S. Knoop, M. Mark, F. Ferlaino, J.G. Danzl, T. Kraemer, H.-C. Nägerl, and R. Grimm
Phys. Rev. Lett. 100, 083002 (2008)


[1] Preparation of a Pure Molecular Quantum Gas
J. Herbig, T. Kraemer, M. Mark, T. Weber, C. Chin, H.-C. Nägerl, and R. Grimm
Science 301, 1510 (2003)

[2] Spectroscopy of ultracold, trapped cesium Feshbach molecules
M. Mark, F. Ferlaino, S. Knoop, J.G. Danzl, T. Kraemer, C. Chin, H.-C. Nägerl, and R. Grimm
Phys. Rev. A 76, 042514 (2007)


This work has been carried out in the LevT experiment.

Steven Knoop (Marie Curie Fellow)
Michael Mark
Francesca Ferlaino (Lise-Meitner Fellow)
Johann Danzl
Tobias Kraemer
Hanns-Christoph Nägerl
Rudi Grimm


The experiment is supported by the Austrian Science Fund (FWF) within No. SFB 15 (project part 16).


last change: 01-03-08 by SK