Deviation from Universality in Collisions of Ultracold [superscript 6] Li[subscript 2] Molecules

Abstract

Collisions of [superscript 6]Li[subscript 2] molecules with free [superscript 6]Li atoms reveal a striking deviation from universal predictions based on long-range van der Waals interactions. Li[subscript 2] closed-channel molecules are formed in the highest vibrational state near a narrow Feshbach resonance and decay via two-body collisions with Li[subscript 2], Li, and Na. For Li[subscript 2]+Li[subscript 2] and Li[subscript 2]+Na, the decay rates agree with the universal predictions of the quantum Langevin model. In contrast, the rate for Li[subscript 2]+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.