Collisions of ${}^6\mathrm{Li}_2$ molecules with free ${}^6\mathrm{Li}$ atoms reveal a striking deviation from universal predictions based on long-range van der Waals interactions. ${\mathrm{Li}}_2$ closed-channel molecules are formed in the highest vibrational state near a narrow Feshbach resonance and decay via two-body collisions with ${\mathrm{Li}}_2$, Li, and Na. For ${\mathrm{Li}}_2+{\mathrm{Li}}_2$ and ${\mathrm{Li}}_2+\mathrm{Na}$, the decay rates agree with the universal predictions of the quantum Langevin model. In contrast, the rate for ${\mathrm{Li}}_2+\mathrm{Li}$ 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.

• Received 16 January 2013

DOI:https://doi.org/10.1103/PhysRevLett.110.173203