Observation of the last bound levels near the Na₂ ground-state asymptote

A multistep laser excitation scheme was used to access the last, least bound levels close to the lowest asymptote 3s+3s with the molecular states X¹Σ_g⁺ and a³Σ_u⁺ of Na₂. All singlet levels within the last 10 cm^-1, and some triplet levels with sufficient singlet admixture, were 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 tunneling through the rotational barrier or electronic state coupling were recorded for levels above the dissociation limit.