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Irradiation damage in lithium-doped silicon gives rise to the well-known Q luminescence-absorption system with no-phonon lines at∼ 1.045 eV. In irradiated lithium-doped material with a high carbon concentration a second luminescence-absorption system is also prominent at higher energy, the S system with no-phonon lines at∼ 1.082 eV. Isotope-splitting data on the no-phonon lines indicate that the S optical center, like the Q center, contains four lithium atoms, but with a near-neighbor carbon atom. Luminescence-decay-time measurements confirm that in both cases the luminescence may be identified with exciton decay at neutral isoelectronic centers and, together with absorption measurements, yield approximate values for the defect concentrations. Zeeman measurements on the Q system show that of the three no-phonon lines observed at zero stress, the higher-energy lines are magnetic singlets and that …