We report on high-resolution spectroscopy of ultracold fermionic
\nak~Feshbach molecules, and identify a two-photon pathway to the rovibrational
singlet ground state via a resonantly mixed \Bcres intermediate state.
Photoassociation in a 23Na-40K atomic mixture and one-photon
spectroscopy on \nak~Feshbach molecules reveal about 20 vibrational levels of
the electronically excited \ctrip state. Two of these levels are found to be
strongly perturbed by nearby \Bsing states via spin-orbit coupling, resulting
in additional lines of dominant singlet character in the perturbed complex
{B1Π∣v=4⟩∼c3Σ+∣v=25⟩}, or of
resonantly mixed character in {B1Π∣v=12⟩∼c3Σ+∣v=35⟩}. The dominantly singlet level is used to locate
the absolute rovibrational singlet ground state X1Σ+∣v=0,J=0⟩ 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, a thousand-fold improvement in
accuracy compared to previous determinations. In their absolute singlet ground
state, \nak~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.Comment: 23 pages, 8 figure
