Observation of the last bound levels near the Na-2 ground-state asymptote

A multistep laser excitation scheme was used to access the last, least boun d levels close to the lowest asymptote 3s + 3s with the molecular states X( 1)Sigma(g)(+) and a 3 Sigma(u)(+) of Na-2. All singlet levels within the la st 10 cm(-1), and some tripler levels with sufficient singlet admixture, we re observed. The level energies were determined with uncertainties of less than 22 MHz with respect to the dissociation limit. The hyperfine structure was resolved, and asymmetric line shapes due to predissociation by tunneli ng through the rotational barrier or electronic state coupling were recorde d for levels above the dissociation limit. [S1050-2947(99)11705-0].