Vibrational levels a few eV below the dissociation asymptote 3s#3p of Na were studied in a molecular beam. The hyperfine structure could be resolved. The molecular levels can be described by adiabatic potentials calculated with the help of atomic parameters: spin-orbit parameter, hyperfine parameter and lifetime of the 3p level. This gives a very precise picture of long range interactions up to internuclear distances R+250 A s . Retardation effects were not identified within the error limits of the data in agreement with theoretical estimates. The life-time derived for the 3p level of the sodium atom agrees very well with independent measurements from different methods.

We report a direct measurement of the Na 2 X 1 ⌺ g ϩ ground-state dissociation energy. Three spectroscopic measurements are combined to directly yield the energy of the lowest (vϭ0) vibrational level relative to the free-atom limit. The X 1 ⌺ g ϩ (vϭ0→31) splitting is measured by double-resonance spectroscopy; X 1 ⌺ g ϩ (v ϭ31)→A 1 ⌺ u ϩ (vЈϭ165) by laser-induced fluorescence; and free atoms→A 1 ⌺ u ϩ (vЈϭ165) by photoassociation of ultracold atoms. The result is D 0 ϭ5942.6880(49) cm Ϫ1 . The uncertainty is a factor of 6 improvement over a previous ͑indirect͒ determination. ͓S1050-2947͑96͒50608-6͔