Singlet [Formula Presented]-wave scattering lengths of [Formula Presented] and [Formula Presented]

Photoassociation of ultracold lithium atoms into bound vibrational levels of the [Formula Presented] excited state is used to probe the [Formula Presented] ground-state interaction potential of [Formula Presented] and [Formula Presented]. It had been predicted that the [Formula Presented]-wave photoassociation signal strength would pass through a minimum as a function of vibrational level for positive [Formula Presented]-wave scattering length. We report the observation of this novel effect, and use the location of the minimum to precisely determine the singlet [Formula Presented]-wave scattering length for both isotopes. The sensitivity of this technique is demonstrated by distinguishing the minima for collisions involving [Formula Presented] atoms in different hyperfine states.