The NaK 1 3 ⌬ state has been studied by the perturbation-facilitated optical-optical double resonance technique. Mixed singlet-triplet levels, A(2) 1 ⌺ ϩ (v A ,J)ϳb(1) 3 ⌸(v b ,J), were pumped from thermally populated rovibrational levels of the ground state, X(1) 1 ⌺ ϩ (v X ,JϮ1), using a single-mode cw dye laser. A single-mode cw Ti:Sapphire laser was then used to further excite the NaK molecules to various 1 3 ⌬(v ⌬ ,N ⌬ ,J ⌬ ) rovibrational levels which were detected by observing

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... nduced 3 ⌳→a(1) 3 ⌺ ϩ fluorescence in the green part of the spectrum. The measured energies of the 1 3 ⌬(v ⌬ ,N ⌬ ) levels were fit to a Dunham expansion, and the Dunham coefficients were used to construct the RKR potential curve. Absolute numbering of the 1 3 ⌬ state vibrational levels was established by a comparison of experimental and calculated 1 3 ⌬(v ⌬ ,N ⌬ ,J ⌬ ) ←b(1) 3 ⌸(v b ,J b ) absorption line strengths. A deperturbation program was used to determine the vibration-dependent 1 3 ⌬ state spin-orbit interaction parameter. Hyperfine structure of the 1 3 ⌬ state was studied, and the Fermi-contact interaction term for this state was determined to be ϳ0.0111 cm Ϫ1 .