Long-range interaction at the asymptote 3s+3p of Na-2

Vibrational levels a few mu eV below the dissociation asymptote 3s + 3p of Na-2 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 approximate to 250 Angstrom. 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.