Rate Saturation of Photoassociation in a Bose-Einstein Condensate

We present the first experimental evidence for saturation of the photoassociation (PA) rate constant, $K_p$, in a Bose-Einstein condensate (BEC). Atomic $^7$Li in the $F$ = 1, $m_F$ = 1 hyperfine state is condensed in an elongated optical trap and a PA pulse coupling the atoms to the $v^\prime$$^\prime$ = 83 vibrational level of the 1 $^3 \Sigma^+_g$ excited molecular state propagates along the long axis of the trap. On-resonant absorption imaging is used to measure the fraction of atoms remaining in the BEC after each PA pulse for a given duration and intensity. $K_p$ is then extracted as a fitting parameter to the time dependent loss. The PA rate is substantially enhanced by tuning to near a magnetic Feshbach resonance at 732G. The rate constant is observed to saturate to a value of $K_p$ $\sim$ 1.4 $\times$ 10 $^-$$^7$ cm$^3$/s for PA pulse intensities $>$ 5 W/cm$^2$. The saturation level is compared with the theories of quantum mechanical unitarity and rogue dissociation.