Singlet s-wave scattering lengths of Li-6 and Li-7

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 Li-6(2) and Li-7(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 Li-7 atoms in different hyperfine states.