Bound-free Emission in the NaK Molecule

We report the analysis of bound-free emission from the $4\,^3\Sigma^+$ electronic state to the $a(1)\,^3\Sigma^+$ repulsive state of the NaK molecule. Taken together with the previous experiments of Burns {\it et al.} [J. Chem.~Phys. {\bfseries 119} 4743--4754 (2003)], recent measurements have provided data for a sufficiently large range of initial vibrational levels (up to $v = 34$ of the $4\,^3\Sigma^+$ state) to probe the behavior of the transition dipole moment function $M(R)$ in the range $R \sim 3.5$ to 6.4~\AA. This range includes a region where theoretical calculations have predicted sharp structure due to an avoided crossing. Using a modified inner repulsive wall for the $a(1)\,^3\Sigma^+$ potential and a refined IPA potential for the $4\,^3\Sigma^+$ state, we have calculated bound free spectra and have adjusted $M(R)$ to fit the experimental data. The work was performed with a version of R. J. Le~Roy's code BCONT that we modified. Preliminary results confirm the sharp structure predicted in $M(R)$, and further work is in progress that will include both bound-free and bound-bound spectra in the analysis.