Two-photon pathway to ultracold ground state molecules of²³Na⁴⁰K

Wereport on high-resolution spectroscopy of ultracold fermionic (NaK)-Na-23-K-40 Feshbach molecules, and identify a two-photon pathway to the rovibrational singlet ground state via a resonantly mixed B-1 Pi similar to c(3)Sigma(+) intermediate state. Photoassociation in a Na-23(-40) Katomic mixture and one-photon spectroscopy on (NaK)-Na-23-K-40 Feshbach molecules reveal about 20 vibrational levels of the electronically excited c3S+ state. Two of these levels are found to be strongly perturbed by nearby B-1 Pi levels via spinorbit coupling, resulting in additional lines of dominant singlet character in the perturbed complex B-1 Pi vertical bar v = 12 > similar to c(3)Sigma(+) vertical bar v = 35 >, or of resonantly mixed character in B-1 Pi vertical bar v = 12 > similar to c(3)Sigma(+) vertical bar v = 35 >. The dominantly singlet level is used to locate the absolute rovibrational singlet ground state X-1 is an element of vertical bar v = 0, J = 0> via Autler-Townes spectroscopy. Wedemonstrate 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, 23 Na 40 Kmolecules 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.