  We present results from two-photon photoassociative spectroscopy of the
least-bound vibrational level of the X$^1\Sigma_g^+$ 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 $^1S_0$-$^3P_1$ 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.
