Two-photon pathway to ultracold ground state molecules of ²³Na⁴⁰K

We report on high-resolution spectroscopy of ultracold fermionic ²³Na⁴⁰K Feshbach molecules, and identify a two-photon pathway to the rovibrational singlet ground state via a resonantly mixed B¹Π ∼ c³Σ⁺intermediate state. Photoassociation in a ²³Na-⁴⁰K atomic mixture and one-photon spectroscopy on ²³Na⁴⁰K Feshbach molecules reveal about 20 vibrational levels of the electronically excited c³Σ⁺state. Two of these levels are found to be strongly perturbed by nearby B¹Π levels via spin-orbit coupling, resulting in additional lines of dominant singlet character in the perturbed complex, or of resonantly mixed character in . The dominantly singlet level is used to locate the absolute rovibrational singlet ground state via Autler-Townes spectroscopy. We demonstrate coherent two-photon coupling via dark state spectroscopy between the predominantly triplet Feshbach molecular state and the singlet ground state. Its binding energy is measured to be 5212.0447(1) cm^-1, a thousand-fold improvement in accuracy compared to previous determinations. In their absolute singlet ground state, ²³Na⁴⁰K molecules are chemically stable under binary collisions and possess a large electric dipole moment of 2.72 Debye. Our work thus paves the way towards the creation of strongly dipolar Fermi gases of NaK molecules.