The excitation and emission spectra of O 2 À molecular ions in KI, KBr and KCl crystals have systematically been investigated at 8.7 K. The present results for the emission spectra with zero-phonon lines were in good agreement with previous results. From the temperature dependence of their emissions, we found that the luminescence intensities of O 2 À ions still appear at room temperature (RT) and are much stronger than those of other chalcogen molecular ions particularly in a KCl crystal. On the other hand, we found vibronic structures on the low-energy sides of the excitation spectra of KI and KBr:O 2 À crystals, showing several peaks with nearly equidistant spacings. The anharmonic vibrational energies of O 2 À molecular ions in the excited state were obtained from these energy differences, depending on their lattice constants. Using spectroscopic constants calculated from the experimental data, we could construct a configurational-coordinate diagram for an O 2 À center in a KI:O 2 À crystal.
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