First experimental observation of the doubly-excited 21Δg state of Na2

We first experimentally observed the doubly-excited electronic state 2¹Δ_g of Na₂ dissociating into Na (3p) + Na (3p) limit. This state has the deepest interaction potential well among the 12 electronic states formed from this first doubly-excited limit. A single line Ar⁺ laser (total of 4 lines) was used to excite the Na₂ molecules from ground state X¹Σ_g⁺ to the intermediate B¹Π_u state. A solid-state pumped single mode Ti:sapphire laser was scanning from 12100 to 14300 cm^-1 to search oh the optical-optical double resonance (OODR) signals. Using a combination of filters and photomultiplier tube to detect the total fluorescence from the OODR, more than 150 ro-vibrational levels from the 2¹Δ_g state were observed. Comparing the detected excitation intensity with the Franck-Condon factors and the observed vibrational separations with theoretical calculations, the lowest observed vibrational quantum number were assigned to ν = 3. The T_e, ω_e, and B_e from Dunham expansion fitting are 32540.669 cm^-1, 124.039 cm^-1 and 0.1186 cm-1, respectively.