It was found that when a KCl crystal doubly doped with a trace of MnS was irradiated with UV light, the S₂^- molecular ions were effectively produced through a photochemical reaction. These S₂^- molecular ions exhibited characteristic vibronic luminescence composed of zero-phonon lines and phonon-side bands below 160 K. Such a vibronic structure related to zero-phonon lines could also be observed for the first time in the luminescence excitation band. From the analysis of the spectral positions of zero-phonon lines in the emission and excitation spectra, the average vibrational frequencies of the S₂^- molecular ion in the ground state and excited state were determined to be 1.78× 10¹³ s^-1 and 1.13× 10¹³ s^-1 respectively.