Abstract
Two-electron model potential calculations are presented for the 4-81 Sigma g+ intermediate Rydberg states of Na2. The theoretical spectroscopic constants agree within 1% with the experimental results of Taylor et al (1981). Two Rydberg series are identified; for one of them, the molecular quantum defect curve reveals a strong resonant character. Diabatic calculations show that the perturbation of this series is mainly due to an excitation of the Na2+ core into the two first excited states.