This report investigates the properties of silicon doped with the transition metal impurities copper and zinc. Low temperature photoluminescence measurements reveal an intense luminescence band with zero phonon structure around 919.56mev.Also contained in the spectra is the characteristic copper-copper pair band which has been identified as originating from isoelectronic Cu-Cu pair centres in a <111> configuration. The 919.56 mev is found to have a slow decay time at low temperatures. This was exploited by the adoption of phase-shift techniques in a phase sensitive lock-in amplifier, which allowed the 919.56 mev band to be studied in isolation from the overlapping copper-copper pair band. The 919.56mev luminescence spectrum exhibits a strong phonon sideband structure. The most prominent zero phonon line is observed at 919.56mev with phonon replicas separated by 6.3 mev indicating strong coupling to a local mode phonon of energy 6.3 mev. Special features observed at higher temperatures around the principle zero phonon line are identified as anti- Stokes replicas as well as lines originating in electron states at = 3.3mev and E^ = 4.6mev above the lower excited state at E . The temperature dependence of the luminescence decay time was studied using deconvolution techniques to correct for the detector response function. A rapid fall off is observed for the decay time over the temperature range 50 - 100K, corresponding to thermalization over an energy barrier of 65 mev. The data is interpreted in terms of an isoelectronic bound exciton. The energy level structure and luminescence decay time are found to agree well with a model in which the binding centre is axial, with a positive (tensile) stress field and a large electron - hole exchange energy of 14 mev. The model proposed for the centre is a + 2 — + Cu - Zn - Cu defect, where a substitutional zinc double acceptor complexes with two interstitial copper atoms to produce the isoelectronic binding centre for the excitons.