  We report a detailed molecular spectroscopy study on the lowest excited
electronic states of $^{23}\rm{Na}^{87}\rm{Rb}$ for producing ultracold
$^{23}\rm{Na}^{87}\rm{Rb}$ molecules in the electronic, rovibrational and
hyperfine ground state. Starting from weakly-bound Feshbach molecules, a series
of vibrational levels of the $A^{1}\Sigma^{+}-b^{3}\Pi$ coupled excited states
were investigated. After resolving, modeling and interpreting the hyperfine
structure of several lines, we successfully identified a long-lived level
resulting from the accidental hyperfine coupling between the $0^+$ and $0^-$
components of the $b^3\Pi$ state, satisfying all the requirements for the
population transfer toward the lowest rovibrational level of the X$^1\Sigma^+$
state. Using two-photon spectroscopy, its binding energy was measured to be
4977.308(3) cm$^{-1}$, the most precise value to date. We calibrated all the
transition strengths carefully and also demonstrated Raman transfer of Feshbach
molecules to the absolute ground state.
