Abstract
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.
Highlights
Ever since laser cooling and evaporative cooling gave full control over the motional and internal degrees of atoms, there has been a strong effort to extend such control over the richer internal structure of molecules
The enormous progress towards ultracold molecules over the last decade has been summarized in review papers [1,2,3], to which one may add the recent successes of magneto-optical trapping [4, 5], a novel type of Sisyphus cooling [6], as well as evaporative cooling of molecules [7], among others
Following our creation of ultracold Feshbach molecules [20], we here perform one- and two-photon spectroscopy on 23Na40K and identify a two-photon pathway from the predominantly triplet Feshbach molecular state to the absolute rovibrational singlet ground state
Summary
Ever since laser cooling and evaporative cooling gave full control over the motional and internal degrees of atoms, there has been a strong effort to extend such control over the richer internal structure of molecules. Following our creation of ultracold Feshbach molecules [20], we here perform one- and two-photon spectroscopy on 23Na40K and identify a two-photon pathway from the predominantly triplet Feshbach molecular state to the absolute rovibrational singlet ground state. Two-photon spectroscopy in the “traditional direction” from the absolute singlet to low-lying triplet ground states of NaK was performed on 23Na39K in a molecular beam [58]. We here demonstrate that even the highest vibrationally excited triplet ground state near a Feshbach resonance can be coherently coupled to the absolute rovibrational singlet ground state with significant coupling strength, enabling the formation of ground state dipolar molecules from ultracold Feshbach molecules. This allows the direct observation of c3Σ+ levels, and thanks to the ultracold temperatures, only low lying rotational states are accessed. Our work opens the door to the creation of a strongly interacting dipolar Fermi gas of 23Na40K molecules
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