Model potential calculations for the intermediate Rydberg states of Na₂: adiabatic potential curves and molecular quantum defects

The excited potential curves of Na₂ are computed up to the states dissociating into Na(3s)+Na(4d) by a two-electron model potential method which uses an expansion on screened two-centre orbitals. The agreement between the computed excitation energies and the experimental determination is better than 200 cm^-1. A good agreement is also found with previous calculations of Jeung (1983,1987) up to the 3s+5s dissociation limit, but not with previous calculations of Jeung and Ross (1988) for the states dissociating into 3p+3p. The analysis of the results in terms of molecular quantum defects reveals some avoided crossings characteristic of two-electron effects.