High Resolution Molecular Spectroscopy for Producing Ultracold Absolute Ground-State ${}^{23}$Na${}^{87}$Rb Molecules

Abstract

We report a detailed molecular spectroscopy study on the lowest excited electronic states of ${}^{23}{\mathrm{Na}}^{87}\mathrm{Rb}$ for producing ultracold ${}^{23}{\mathrm{Na}}^{87}\mathrm{Rb}$ molecules in the electronic, rovibrational and hyperfine ground state. Starting from weakly-bound Feshbach molecules, a series of vibrational levels of the $A^1{\Sigma }^{+}-b^3\Pi$ coupled excited states were investigated. After resolving, modeling and interpreting the hyperfine structure of several lines, we successfully identified a long-lived level resulting from the accidental hyperfine coupling between the $0^{+}$ and $0^{-}$ components of the $b^3\Pi$ state, satisfying all the requirements for the population transfer toward the lowest rovibrational level of the X${}^1{\Sigma }^{+}$ state. Using two-photon spectroscopy, its binding energy was measured to be 4977.308(3) cm${}^{-1}$, the most precise value to date. We calibrated all the transition strengths carefully and also demonstrated Raman transfer of Feshbach molecules to the absolute ground state.

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