The X$^{1}Sigma ^{+}$ state of NaRb was studied by Fourier transform spectroscopy. An accurate potential energy curve was derived from more than 8800 transitions in isotopomers $^{23}$Na$^{85}$Rb and $^{23}$Na$^{87}$Rb. This potential reproduces the experimental observations within their uncertainties of 0.003 rcm to 0.007 rcm. The outer classical turning point of the last observed energy level ($v=76$, $J=27$) lies at $approx 12.4$ AA, leading to a energy of 4.5 rcm below the ground state asymptote.
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