Potential of the ground state of NaRb

The $X\phantom{\rule{0.3em}{0ex}}^{1}\ensuremath{\sum}^{+}$ state of NaRb was studied by Fourier-transform spectroscopy. An accurate potential energy curve was derived from more than 8800 transitions in isotopomers $^{23}\mathrm{Na}^{85}\mathrm{Rb}$ and $^{23}\mathrm{Na}^{87}\mathrm{Rb}$. This potential reproduces the experimental observations within their uncertainties of $0.003\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{\ensuremath{-}1}\char21{}0.007\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$. The outer classical turning point of the last observed energy level $({v}^{\ensuremath{''}}=76,\phantom{\rule{0.2em}{0ex}}{J}^{\ensuremath{''}}=27)$ lies at $\ensuremath{\approx}12.4\phantom{\rule{0.3em}{0ex}}\mathrm{\AA{}}$, leading to an energy of $4.5\phantom{\rule{0.3em}{0ex}}{\text{cm}}^{\ensuremath{-}1}$ below the ground-state asymptote.