Singlets-wave scattering lengths ofLi6andLi7

Photoassociation of ultracold lithium atoms into bound vibrational levels of the $A{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}$ excited state is used to probe the $X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ ground-state interaction potential of ${}_{}{}^6{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_2^{}{}_{}{}^{}$. It had been predicted that the $s$-wave photoassociation signal strength would pass through a minimum as a function of vibrational level for positive $s$-wave scattering length. We report the observation of this novel effect, and use the location of the minimum to precisely determine the singlet $s$-wave scattering length for both isotopes. The sensitivity of this technique is demonstrated by distinguishing the minima for collisions involving ${}_{}{}^7{}_{}{}^{}\mathrm{Li}$ atoms in different hyperfine states.