Observation of Hyperfine Structure of the 2³II_1g State in Na₂

We reported the results of the hyperfine structure (HFS) of the 2³II_1g state in Na₂. The relative ultraviolet and visible fluorescence spectra of molecular sodium were measured by means of Doppler-free two-photon spectroscopy technique. The HFS of the 2³II_1g(v = 78, J = 37, Ω = 1) level was first observed and its HFS constant was measured to be C = -4.92±0.26MHz. The intermediate enchancing and upper energy levels were labled by the experimental and theoretical methods.