Two-photon photoassociative spectroscopy of ultracold 88-Sr

We present results from two-photon photoassociative spectroscopy of the least-bound vibrational level of the X${}^1{\mathrm{\Sigma }}_g^{+}$ state of the ${}^{88}$Sr${}_2$ dimer. Measurement of the binding energy allows us to determine the s-wave scattering length, $a_{88}=-1.4(6)a_0$. For the intermediate state, we use a bound level on the metastable ${}^1{S}_0$-${}^3{P}_1$ potential, which provides large Franck-Condon transition factors and narrow one-photon photoassociative lines that are advantageous for observing quantum-optical effects such as Autler-Townes resonance splittings.