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.
Categories: physics.atom-ph