Optical-optical double resonance spectroscopy of the 4 ¹Σ _g⁺ "shelf" state of Na₂ using an ultrasensitive ionization detector

The 4 ¹Σ_g⁺ "shelf" state of Na₂ was extensively investigated using optical-optical double resonance spectroscopy and an ultrasensitive ionization detector. A total of 179 vibrational levels-(0≤v≤127 are from previous work by using filter-photomultiplier tube fluorescence detection) were observed in this state, which covered more than 99.8% of the well depth. The Rydberg-Klein-Rees (RKR) potential energy curve was constructed with maximum outer turning point R ¹⁷⁸⁺ = 17.60 Å. The comparison of the long range portion of the RKR potential energy curve and the potential curve calculated from V(R) = Σ_nC_n/Rⁿ is discussed. Avoided crossings around the shelf with the 3 ¹Σ_g⁺ state and near dissociation with the 5 ¹Σ_g⁺ state are also discussed.