Molecular beam study of thea3Σ+state of NaK up to the dissociation limit

Abstract

We provide spectroscopic data for the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ state of the $^{23}\mathrm{Na}^{39}\mathrm{K}$ molecule. The experiment is done in an ultrasonic beam apparatus, starting from the ground state $X{\phantom{\rule{0.16em}{0ex}}}^{1}{\ensuremath{\Sigma}}^{+}$ and driving the population to the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ state, using a $\ensuremath{\Lambda}$ scheme with fixed pump and scanning dump laser. The signals are observed as dips of the total fluorescence. The intermediate level is chosen to be strongly perturbed by the $B{\phantom{\rule{0.16em}{0ex}}}^{1}\ensuremath{\Pi}/c{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ states mixing to overcome the singlet-triplet transfer prohibition. We observed highly resolved hyperfine spectra of various rovibrational levels of the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ state from ${v}_{a}=2$ up to the highest vibrational levels for rotational quantum numbers ${N}_{a}=4,6,8$. By the typical experimental linewidth of 17 MHz, the vibrational dependence of the hyperfine splitting is clearly revealed for NaK. The absolute frequency measurements of the vibrational levels are used for improvement of the $a{\phantom{\rule{0.16em}{0ex}}}^{3}{\ensuremath{\Sigma}}^{+}$ potential curve and of the derived scattering length of all natural isotope combinations. Applying the $\ensuremath{\Lambda}$ scheme in the reverse direction can provide a pathway for efficient transfer of ultracold $^{23}\mathrm{Na}^{39}\mathrm{K}$ molecules from the $\text{Na}(3s)+\text{K}(4s)$ asymptote to the lowest levels of the ground state. We show spectra that couple the absolute ground state ${v}_{X}=0,J=0$ with an appropriate intermediate state for direct realization of the reverse path. The refined theoretical model of the coupled excited states of the $\text{Na}(3s)+\text{K}(4p)$ asymptote allows predictions of efficient paths for $^{23}\mathrm{Na}^{40}\mathrm{K}$; one example is calculated.