Nonadiabatic Coupling in the $3\,^3\Pi$ and $4\,^3\Pi$ State of NaK

The excited $3\,^3\Pi$ and $4\,^3\Pi$ electronic states of the NaK molecule exhibit an avoided crossing, leading to anomalous behavior of many features of the ro-vibrational energy levels belonging to each state. We have carried out a joint experimental and theoretical investigation of these states. Experimentally, the hyperfine structure of numerous ro-vibrational levels has been determined using the Doppler-free, perturbation-facilitated optical-optical double resonance (PFOODR) technique. Striking patterns in the data provide a sensitive probe of the electronic wave function in the various regions of the double well $3\,^3\Pi$ potential. Companion {\it ab initio} electronic structure calculations have provided adiabatic and diabatic potential curves that account for the avoided crossing. The nonadiabatic coupling between the $3\,^3\Pi$ and $4\,^3\Pi$ states can be exactly formulated in terms of the diabatic potential curves. Using the {\it ab initio} diabatic potential curves as a starting point, we used a nonlinear fitting routine to adjust the potential curves to fit the observed structure. The pertubations between the $3\,^3\Pi$ and $4\,^3\Pi$ states have been accurately reproduced.