Photoassociation of ultracold lithium atoms into bound vibrational lev
els of the A (1) Sigma(u)(+) excited state is used to probe the X(1) S
igma(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 p
ositive s-wave scattering length. We report the observation of this no
vel effect, and use the location of the minimum to precisely determine
the singlet s-wave scattering length for both isotopes. The sensitivi
ty of this technique is demonstrated by distinguishing the minima for
collisions involving Li-7 atoms in different hyperfine states.