A pathway to ultracold bosonic $^{23}\textrm{Na}^{39}\textrm{K}$ ground state molecules

New J. Phys. 21 (2019) 123034 We spectroscopically investigate a pathway for the conversion of $^{23}\textrm{Na}^{39}\textrm{K}$ Feshbach molecules into rovibronic ground state molecules via STImulated Raman Adiabatic Passage (STIRAP). Using photoassociation spectroscopy from the diatomic scattering threshold in the $a^3\Sigma^+$ potential, we locate the resonantly mixed electronically excited intermediate states $|B^1\Pi, v=8\rangle$ and $|c^3\Sigma^+, v=30\rangle$ which, due to their singlet-triplet admixture, serve as an ideal bridge between predominantly $a^3\Sigma^+$ Feshbach molecules and pure $X^1\Sigma^+$ ground state molecules. We investigate their hyperfine structure and present a simple model to determine the singlet-triplet coupling of these states. Using Autler-Townes spectroscopy, we locate the rovibronic ground state of the $^{23}\textrm{Na}^{39}\textrm{K}$ molecule ($|X^1\Sigma^+, v=0, N=0\rangle$) and the second rotationally excited state $N=2$ to unambiguously identify the ground state. We also extract the effective transition dipole moment from the excited to the ground state. Our investigations result in a fully characterized scheme for the creation of ultracold bosonic $^{23}\textrm{Na}^{39}\textrm{K}$ ground state molecules.