  We demonstrate the transfer of $^{23}$Na$^{40}$K molecules from a
closed-channel dominated Feshbach-molecule state to the absolute ground state.
The Feshbach molecules are initially created from a gas of sodium and potassium
atoms via adiabatic ramping over a Feshbach resonance at 78.3$\,$G. The
molecules are then transferred to the absolute ground state using stimulated
Raman adiabatic passage with an intermediate state in the spin-orbit-coupled
complex $|c^3 \Sigma^+, v=35, J=1 \rangle \sim |B^1\Pi, v=12, J=1\rangle$. Our
measurements show that the pump transition dipole moment linearly increases
with the closed-channel fraction. Thus, the pump-beam intensity can be two
orders of magnitude lower than is necessary with open-channel dominated
Feshbach molecules. We also demonstrate that the phase noise of the Raman
lasers can be reduced by filter cavities, significantly improving the transfer
efficiency.
