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arXiv:1301.3566 · Physical Review Letters

Deviation from Universality in Collisions of Ultracold Li26 Molecules

Tout T. Wang, Myoung-Sun Heo, Timur M. Rvachov, Dylan A. Cotta, Wolfgang Ketterle

Published Apr 25, 2013• 19 citations
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Abstract

Collisions of 6Li2 molecules with free 6Li atoms reveal a striking deviation from universal predictions based on long-range van der Waals interactions. Li2 closed-channel molecules are formed in the highest vibrational state near a narrow Feshbach resonance and decay via two-body collisions with Li2, Li, and Na. For Li2 + Li2 and Li2 + Na, the decay rates agree with the universal predictions of the quantum Langevin model. In contrast, the rate for Li2 + Li is exceptionally small, with an upper bound 10 times smaller than the universal prediction. This can be explained by the low density of available decay states in systems of light atoms [G. Quéméner, J.-M. Launay, and P. Honvault, Phys. Rev. A 75, 050701 (2007)], for which such collisions have not been studied before.

DOI
10.1103/physrevlett.110.173203

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We have performed a quantum-dynamical study of vibrational deexcitation and elastic scattering at ultralow collision energies for the bosonic system 7Li+7Li2^{7}\mathrm{Li}+^{7}\mathrm{Li}_{2}7Li+7Li2​ and for the fermionic system 6Li+6Li2^{6}\mathrm{Li}+^{6}\mathrm{Li}_{2}6Li+6Li2​ with diatomic molecules in high vibrational states. Inelastic rates show a strong and irregular dependence on the vibrational state of the diatomic molecule. For the highest vibrational state which lies near the dissociation limit, vibrational deexcitation processes are not suppressed, but rather depleted, for both bosonic and fermionic systems.
We compare and contrast the wide Feshbach resonances and the corresponding weakly bound states in the lowest scattering channels of ultracold 6Li^{6}\mathrm{Li}6Li and 7Li^{7}\mathrm{Li}7Li. We use high-precision measurements of binding energies and scattering properties to determine interaction potentials that incorporate non-Born-Oppenheimer terms to account for the failure of mass scaling between 6Li^{6}\mathrm{Li}6Li and 7Li^{7}\mathrm{Li}7Li. Correction terms are needed for both the singlet and the triplet potential curves. The universal formula relating binding energy to scattering length is not accurate for either system. The 6Li^{6}\mathrm{Li}6Li resonance is open-channel-dominated and the van der Waals formula of Gao [J. Phys. B 37, 4273 (2004)] gives accurate results for the binding energies across much of the resonance width. The 7Li^{7}\mathrm{Li}7Li resonance, by contrast, is weakly closed-channel-dominated and a coupled-channel treatment of the binding energies is required. Plotting the binding energies in universal van der Waals form helps illustrate subtle differences between the experimental results and different theoretical forms near the resonance pole.
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