Fine and hyperfine structure of ro-vibrational levels of the NaK $1\,^{1,3}\!\Delta$ states near the dissociation limit

Our previous high-resolution spectroscopic studies of the fine and hyperfine structure of ro-vibrational levels of the $1\,^3\!\Delta$ state of NaK have been extended to include vibrational levels up to $v = 59$, the hightest of which are within $8\,\mathrm{cm}^{-1}$ of the dissociation limit. Using the IPA method, a potential curve is determined that reproduces all measured levels ($35 \le v \le 59$) to an accuracy of $\sim 0.026\,\mathrm{cm}^{-1}$, and $C_6$ and $C_8$ coefficients have also been determined from the long range potential. The fine and hyperfine structure of the $1\,^3\!\Delta$ ro-vibrational levels has been fit using a theoretical model that treats the intermediate angular momentum coupling, leading to values $A_v$ and $b_{\mathrm{F}}$ of the spin-orbit coupling constant and the hyperfine Fermi contact constant. The measured fine and hyperfine structure for $v$ in the range $42 \le v \le 48$ exhibits anomalous behavior whose origin is believed to be the mixing between the $1\,^3\!\Delta$ and $1\,^1\!\Delta$ states. The theoretical method has been extended to treat this interaction, and the results provide an accurate representation of the complicated patterns that arise.