Study of the 41Σ_g⁺ "shelf" state of Na₂ by optical-optical double resonance spectroscopy

Due to an avoided crossing, the 4¹Σ⁺ state of Na₂ has an unusual potential energy curve with a "shelf" on its outer wall. By using optical-optical double resonance techniques, vibrational levels from υ = 0 to 130 have been observed and the shelf was found around υ = 48-60. From the experimental data we have successfully constructed a Rydberg-Klein-Rees (RKR) potential curve, which can reproduce measured data with a standard deviation of 0.064 cm^-1. A set of Dunham coefficients was obtained for the lower part υ = 0-45 of that state. Several interesting effects related to the shelf, such as the unusual behavior of B_υ and D_υ, of the vibrational wave functions and the semiclassical vibrational periods, and of the rotational effective potential, are discussed.