We report a photoluminescence (PL) study of silicon subject to mercury-ion implantation and subsequent annealing A luminescence center is observed with a characteristic bound-exciton (BE) spectrum consisting of a narrow no-phonon line at 1067.9 meV and a series of equidistant phonon replicas with a phonon energy of about 8 meV. The effects of the various external perturbations in the photoluminescence experiments, such as variations in temperature and excitation power, are reported. The transient PL results give a thermal activation energy (25 meV) consistent with the temperature dependence of the PL intensity. All PL lines are shown to arise from a single isoelectronic center