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We present a detailed study of interspecies Feshbach resonances of the bosonic $^{23}\mathrm{Na}+^{39}\mathrm{K}$ mixture for magnetic fields up to $750\phantom{\rule{0.16em}{0ex}}\mathrm{G}$ in various collision channels. A total of 14 Feshbach resonances are reported, as well as four zero crossings of the scattering length and three inelastic two-body loss features. We use the observed magnetic-field locations of the resonant features together with the known data on $^{23}\mathrm{Na}+^{40}\mathrm{K}$ to refine the singlet and triplet ground-state potentials of NaK and achieve a consistent description of Feshbach resonances for both the Bose-Bose mixture of $^{23}\mathrm{Na}+^{39}\mathrm{K}$ and the Bose-Fermi mixture of $^{23}\mathrm{Na}+^{40}\mathrm{K}$. We also discuss the influence of the interplay between inelastic two-body and three-body processes on the observation of a Feshbach resonance.

We report an extensive series of transitions (including collisional transfer lines) from pure and mixed levels of the NaK ${A}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ and $b\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Pi}}$ states to the ${X}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ state, observed at the Universit\'e Lyon 1 using Fourier-transform spectroscopy. We then combine these data with previously reported data on these states from emission from the ${B}^{1}\mathrm{\ensuremath{\Pi}}$ and ${C}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}$ states and from mutually perturbed levels of the $D{\phantom{\rule{0.16em}{0ex}}}^{1}\mathrm{\ensuremath{\Pi}}$ and $d\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Pi}}$ states. We obtain 2758 distinct term values: the full data set includes 11 624 term values, with many multiple determinations from transitions over a range of vibrational and rotational levels. The data are analyzed by fitting to potentials of the ``Hannover'' form [C. Samuelis et al., Phys. Rev. A 63, 012710 (2000)] plus spin-orbit (SO) functions in a simple Morse form, yielding an rms residual of approximately 0.029 ${\mathrm{cm}}^{\ensuremath{-}1}$. The empirical SO functions agree well with their ab initio counterparts obtained from electronic structure calculations based on nonempirical effective core potentials. From level energies of the $A\ensuremath{-}b$ complex calculated from the fitted potentials and SO functions, we identify reasonable candidates for transitions between Feshbach resonance states and mixed singlet-triplet gateway levels of the ${A}^{1}{\mathrm{\ensuremath{\Sigma}}}^{+}\ensuremath{-}b\phantom{\rule{0.16em}{0ex}}^{3}\mathrm{\ensuremath{\Pi}}$ manifold, leading either to $v=0$ levels of the $X$ state or to mixed singlet-triplet levels at higher energies that can be used for perturbation-facilitated double-resonance experiments.

The $3{}^{1}\ensuremath{\Pi}$ and $6{}^{1}{\ensuremath{\Sigma}}^{+}$ states of the NaK molecule were investigated by polarization labeling spectroscopy in a heat pipe oven. About 2000 molecular transitions in the $3{}^{1}\ensuremath{\Pi}\ensuremath{\leftarrow}X{}^{1}{\ensuremath{\Sigma}}^{+}$ and $6{}^{1}{\ensuremath{\Sigma}}^{+}\ensuremath{\leftarrow}X{}^{1}{\ensuremath{\Sigma}}^{+}$ band systems were measured in the $24600\char21{}27700{\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ spectral range. The analysis provided molecular constants and the potential-energy curve corresponding to 97% of the potential well of the $3{}^{1}\ensuremath{\Pi}$ state. For the double minimum $6{}^{1}{\ensuremath{\Sigma}}^{+}$ state nearly the whole inner potential well was characterized. The results are compared with the predictions of recent theoretical calculations.

Potential curves of the ${\mathrm{K}}_{2}$ and NaK molecules have been computed in the framework of pseudopotential methods, over a wide range of interatomic distances. At short internuclear distances, the agreement with experimental data is excellent, the mean deviation between theoretical and experimental spectroscopic constants being 1% for the two alkali dimers. At large interatomic separation, wells and avoided crossings are observed in adiabatic potential curves of highly excited states correlated to the asymptotes close to K[4p]+K[4p] and Na[3p]+K[4p]. For example, the well depths of the $^{1}\mathrm{\ensuremath{\Sigma}}^{+}$ states reach 2650 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ for ${\mathrm{K}}_{2}$ and 4330 ${\mathrm{cm}}^{\mathrm{\ensuremath{-}}1}$ in the case of NaK. We demonstrate that they correspond to pseudocrossings between covalent states (dissociating into K[nl]+K[n\ensuremath{'}l\ensuremath{'}] or Na[nl]+K[n\ensuremath{'}l\ensuremath{'}]) and ionic states (correlated to ${\mathrm{K}}^{+}$+${\mathrm{K}}^{\mathrm{\ensuremath{-}}}$ for ${\mathrm{K}}_{2}$, ${\mathrm{Na}}^{+}$+${\mathrm{K}}^{\mathrm{\ensuremath{-}}}$, or ${\mathrm{Na}}^{\mathrm{\ensuremath{-}}}$+${\mathrm{K}}^{+}$ for NaK, the negative ion being in the ground state or an autoionizing state). As for ${\mathrm{Na}}_{2}$, these structures may play a crucial role in the interpretation of low-energy collisions. \textcopyright{} 1996 The American Physical Society.

We have measured the ${\mathit{X}}^{1}$${\mathrm{\ensuremath{\Sigma}}}^{+}$--B $^{1}\mathrm{\ensuremath{\Pi}}$ bound-free molecular photodissociation profile of NaK under molecular-beam conditions. The dissociation results in excited ${\mathrm{K}}^{\mathrm{*}}$(4 $^{2}$P). We have measured the ${\mathit{D}}_{1}$/${\mathit{D}}_{2}$ fine-structure branching ratio; this ratio is a strong function of wavelength ranging from near zero to ${\mathit{D}}_{1}$/${\mathit{D}}_{2}$\ensuremath{\simeq}25% as the laser is tuned through the bound-free absorption profile.