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 $^3Pi_{u}$ and $^3Sigma^+ _{u}$ 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.
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