Deviation from Universality in Collisions of Ultracold ${}^6$Li${}_2$ Molecules

Abstract

Collisions of ${}^6$Li${}_2$ molecules with free ${}^6$Li atoms reveal a striking deviation from universal predictions based on long-range van der Waals interactions. Li${}_2$ closed-channel molecules are formed in the highest vibrational state near a narrow Feshbach resonance, and decay via two-body collisions with Li${}_2$, Li, and Na. For Li${}_2$+Li${}_2$ and Li${}_2$+Na, the decay rates agree with the universal predictions of the quantum Langevin model. In contrast, the rate for Li${}_2$+Li is exceptionally small, with an upper bound ten 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 \textbf{75}, 050701 (2007)], for which such collisions have not been studied before.

5 pages, 5 figures

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