B 1Π→A 1Σ+ ELECTRONIC TRANSFER IN NaK

begin{wrapfigure}[13]{r}{0pt} _x000d_ includegraphics[scale=0.29]{NaK_graphicalAbstract.eps}_x000d_ end{wrapfigure}_x000d_ We investigate collisionally-induced ${emph{A}~^1Sigma^+ rightarrow emph{X}~^1 Sigma^+}$ fluorescence in NaK, observed _x000d_ following ro-vibrationally selective excitation of the $emph{B}~^1Pi$ state. _x000d_ NaK molecules are formed in a heatpipe oven, and excited with a single-mode dye laser operating around 17000 {cm$^{-1}$}. _x000d_ Direct ${emph{B};^1Pirightarrow emph{X};^1 Sigma^+}$ fluorescence is dominated by $Delta J =$0 or $pm1$ transitions, with rotational _x000d_ satellites whose intensities vary according to collisional partner (rare gas or K atoms). _x000d_ The emph{B}$rightarrow$emph{X} emission also shows even weaker ${Delta v pm1} $ vibrational relaxation bands,_x000d_ with a more even spread of rotational population. Some 5000 {cm$^{-1}$} to lower wavenumber, we observe apparently_x000d_ non-selective ${emph{A};^1Sigma^+ rightarrow emph{X};^1 Sigma^+}$ emission, with an oscillating bound-free ${emph{c};^3Sigma^+ rightarrow emph{a};^3 Sigma^+}$ contribution to the baseline that becomes increasingly important as higher vibrational levels of $emph{B};^1Pi $ are populated by the laser._x000d_ Earlier work defining analytical functions describing the $emph{A};^1Sigma^+$ and $emph{b};^3Pi$ _x000d_ states of NaK, and the spin-orbit functions coupling themfootnote{Harker emph{et al.}, Phys. Rev. A 92 (1), 012506 (2015)}, _x000d_ allowed us to assign this dense and irregular $emph{A}rightarrowemph{X}$ spectrum, and to see how upper state level populations change as a function of emph{B} state excitation and heatpipe conditions (He, Ar or N$_2$ as buffer gas). _x000d_ Our spectra reveal a propensity to conserve _x000d_ vibrational quantum number, at least from low $v$ in the emph{B} state. This is in contradiction with emph{A-X} emission in _x000d_ Na$_2$ observedfootnote{Astill emph{et al.}, Chem. Phys. Lett. _x000d_ underline{125} 33 (1986); Camacho emph{et al.}, J. Phys B. underline{39} 2665 (2006)} following laser excitation of the (corresponding) $emph{B}_x000d_ ;^1Pi_u$ state. The non-Boltzmann populations in the emph{A};$^1 Sigma_u^+$ state of Na$_2$ were explainedfootnote{Hussein emph{et al.}, J. Mol Spectrosc. underline{114} 105 (1985)} by near-resonant transfer from $emph{B}_x000d_ ;^1Pi_u$ to $emph{2};^1Sigma_g^+$ (with no counterpart in heteronuclear NaK), followed by spontaneous_x000d_ emission to the emph{A};$^1Sigma_u^+$ state