Experimental and theoretical studies of the coupled A 1 Σ + and b Π 3 states of NaK

We report an extensive series of transitions (including collisional transfer lines) from pure and mixed levels of the NaK A1Σ+ and b3Π states to the X1Σ+ state, observed at the Université Lyon 1 using Fourier-transform spectroscopy. We then combine these data with previously reported data on these states from emission from the B1Π and C1Σ+ states and from mutually perturbed levels of the D1Π and d3Π 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 cm−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−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 A1Σ+−b3Π 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.