In this work we model and realize stimulated Raman adiabatic passage (STIRAP) in the diatomic 23 Na 40 K molecule from weakly bound Feshbach molecules to the rovibronic ground state via the |v d = 5, J = Ω = 1 excited state in the d 3 Π electronic potential. We demonstrate how to set up a quantitative model for polar molecule production by taking into account the rich internal structure of the molecules and the coupling laser phase noise. We find excellent agreement between the model predictions and the experiment, demonstrating the applicability of the model in the search of an ideal STIRAP transfer path. In total we produce 5000 fermionic groundstate molecules. The typical phase-space density of the sample is 0.03 and induced dipole moments of up to 0.54 Debye could be observed.Dipolar quantum gases allow for the realization of intriguing new quantum many-body systems and associated phenomena due to their anisotropic and long-range interactions. Among these are the roton driven fluid to crystalline quantum phase transition [1], dipolar droplet formation [2,3], insulators with fractional filling and supersolid phases of dipoles in optical lattices [4] to name only a few. Ultracold polar molecules promise particularly large dipolar interactions due to their large dipole moments.The standard procedure for creating molecules at high phase-space density starts with a mixture of two atomic species close to quantum degeneracy. The two species are then initially adiabatically associated into a weakly bound Feshbach molecular state |F B [5]. From there they can be transferred into the final, electronic, vibrational and rotational (rovibronic) ground state using stimulated Raman adiabatic passage (STIRAP) [6,7]. This last step involves coupling the initial and final state to a common intermediate, electronically excited molecular state. Both |F B and the intermediate state need to be chosen with care in order to allow for a high efficiency in the transfer and thus to preserve the phase space density of the ultracold mixture. This approach has been applied successfully to dipolar KRb [8] to the ground state compared to the STIRAP scheme employed in [11]. We develop a Hamiltonian model to describe the adiabatic transfer in all required details to achieve a quantitative description. In addition to the molecular structure analysis done for different bialkali systems [14,15] we include the complex light coupling into the analysis. This results in a multi-level, crosscoupled model that is intimately related to the work of the Bergmann group on STIRAP in multilevel systems [16] but is specific to the alkali-alkali molecule formation. We investigate how to maximize the STIRAP transfer efficiency for a given intermediate state manifold by optimizing pulse durations and one-photon detuning. We find excellent agreement between simulation and experiment. Finally, we demonstrate ground state molecule creation with a large electric dipole moment of up to 0.54 Debye. I. MOLECULAR LEVEL STRUCTURE AND HAMILTONIAN MODELIn our model we us...
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