  By two-color photoassociation of $^{40}$Ca four weakly bound vibrational
levels in the Ca$_2$ \Xpot ground state potential were measured, using highly
spin-forbidden transitions to intermediate states of the coupled system
$^3\Pi_{u}$ and $^3\Sigma^+ _{u}$ near the ${^3P_1}$+${^1S_0}$ asymptote. From
the observed binding energies, including the least bound state, the long range
dispersion coefficients $\mathrm{C}_6, \mathrm{C}_8,\mathrm{C}_{10}$ and a
precise value for the s-wave scattering length of 308.5(50)~$a_0$ were derived.
From mass scaling we also calculated the corresponding scattering length for
other natural isotopes. From the Autler-Townes splitting of the spectra, the
molecular Rabi frequency has been determined as function of the laser intensity
for one bound-bound transition. The observed value for the Rabi-frequency is in
good agreement with calculated transition moments based on the derived
potentials, assuming a dipole moment being independent of internuclear
separation for the atomic pair model.
