OBSERVATION OF SOME Ω = 1/2 ELECTRONIC STATES OF NICKEL DEUTERIDE, NiD, WITH LASER-INDUCED FLUORESCENCE

\begin{wrapfigure}{l}{0pt} \includegraphics[scale=0.16]{GraphAbs.eps} \end{wrapfigure} The five lowest-lying electronic states of nickel hydride (NiH) are usually labeled $^2\Delta_{5/2}$, $^2\Pi_{3/2}$, $^2\Delta_{3/2}$, $^2\Sigma^+_{1/2}$ and $^2\Pi_{1/2}$, although there is significant mixing between them. These states arise from the $d^9$ electron configuration of Ni$^+$, perturbed by an H$^-$ ligand. A variety of vibrational levels has been observed in each, and the aggregate data set has been well modelled as a `supermultiplet' by the Field group\footnote{J. A. Gray, M. Li, T. Nelis and R. W. Field, J. Chem. Phys. \textbf{95}, 7164 (1991)}. For the deuterated isotopologue NiD, only the $^2\Delta_{5/2}$, $^2\Pi_{3/2}$ and $^2\Delta_{3/2}$ states have been reported in the literature. A multi-isotope supermutiplet fitting including both the NiH and (more limited) NiD data\footnote{M. Abbasi, A. Shayesteh, P. Crozet and A. J. Ross, J. Mol. Spectrosc. \textbf{349}, 49 (2018)} provided predictions for the two $\Omega=1/2$ states of the NiD supermultiplet. Experimental observation was needed to validate (and improve) the model. We report on laser-induced fluorescence experiments conducted both at the University of New Brunswick and at Universit\'e Lyon 1 in which the $^2\Sigma^+_{1/2}, v=0,1,2$ and $^2\Pi_{1/2},v=0,1$ levels of NiD were identified and rotationally analyzed. The existing multi-isotope supermultiplet model proved remarkably accurate in predicting the energy and structure of these $\Omega=1/2$ states. In addition, a higher-lying $\Omega=1/2$ electronic state [16.7]0.5 has been identified in NiD, with no obvious analogue in NiH. The [16.7]0.5-$^2\Sigma^+_{1/2}$ and [16.7]0.5-$^2\Pi_{1/2}$ transitions proved to be a rich source of information about the two lower states