We employ two-photon spectroscopy to study the vibrational states of the triplet ground state potential (a³Σ⁺) of the ²³Na⁶Li 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 a³Σ⁺ potential are reported. We also observe rotational splitting in the lowest vibrational state.