  We perform two-photon photoassociation spectroscopy of the heteronuclear CsYb
molecule to measure the binding energies of near-threshold vibrational levels
of the $X~^{2}\Sigma_{1/2}^{+}$ molecular ground state. We report results for
$^{133}$Cs$^{170}$Yb, $^{133}$Cs$^{173}$Yb and $^{133}$Cs$^{174}$Yb, in each
case determining the energy of several vibrational levels including the
least-bound state. We fit an interaction potential based on electronic
structure calculations to the binding energies for all three isotopologs and
find that the ground-state potential supports 77 vibrational levels. We use the
fitted potential to predict the interspecies s-wave scattering lengths for all
seven Cs+Yb isotopic mixtures.
