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$^1\Sigma_g^+$ 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. 
Y. N. Martinez de Escobar, P. G. Mickelson, P. Pellegrini, S. B.   Nagel, A. Traverso, M. Yan, R. Cote and T. C. Killian