Mixing of ro-vibrational levels of the $3\,^3\Pi$ and $4\,^3\Pi$ states of NaK due to nonadiabatic coupling

We report further results of our investigation of the excited $3\,^3\Pi$ and $4\,^3\Pi$ electronic states of NaK. These electronic states exhibit an avoided crossing, and many ro-vibrational levels are mixtures of both electronic components. The hyperfine structure and energies of numerous ro-vibrational levels were determined using the Doppler-free, perturbation-facilitated optical-optical double resonance (PFOODR) technique [J.~Chem.~Phys.\ {\bfseries 122}, 144313 (2005)]. However, the hyperfine spectra of the $3\,^3\Pi$ and $4\,^3\Pi$ levels are very similar, and excitation spectra to the two states are difficult to distinguish. Further work has shown that bound-free spectroscopy provides a means of differentiating the two electronic states and even estimating the mixing fractions for pairs of ro-vibrational levels. The nonadiabatic coupling between the $3\,^3\Pi$ and $4\,^3\Pi$ states has been exactly formulated in terms of the diabatic potential curves, and the diabatic curves have been determined that best fit the experimental data. Comparison between experimental and theoretical mixing fractions will be presented.