  We employ two-photon spectroscopy to study the vibrational states of the
triplet ground state potential ($a^3\Sigma^+$) of the $^{23}$Na$^{6}$Li
molecule. Pairs of Na and Li atoms in an ultracold mixture are photoassociated
into an excited triplet molecular state, which in turn is coupled to
vibrational states of the triplet ground potential. Vibrational state binding
energies, line strengths, and potential fitting parameters for the triplet
ground $a^3\Sigma^+$ potential are reported. We also observe rotational
splitting in the lowest vibrational state.
