Direct measurement of the ground-state dissociation energy ofNa2

We report a direct measurement of the ${\mathrm{Na}}_2$ $X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}$ ground-state dissociation energy. Three spectroscopic measurements are combined to directly yield the energy of the lowest ($v=0\mathrm{}$) vibrational level relative to the free-atom limit. The $X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}(v=0\mathrm{}\to 31)$ splitting is measured by double-resonance spectroscopy; $X{}_{}{}^1{}_{}{}^{}\Sigma _g^{+}{}_{}{}^{}\mathrm{}(v=31\mathrm{})\mathrm{}\to A{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}(v^{\prime }=165)$ by laser-induced fluorescence; and free $\mathrm{atoms}\to A{}_{}{}^1{}_{}{}^{}\Sigma _u^{+}{}_{}{}^{}(v^{\prime }=165)$ by photoassociation of ultracold atoms. The result is $D_0=5942.6880\mathrm{}\left(49\right)\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. The uncertainty is a factor of 6 improvement over a previous (indirect) determination.