In this work we have been able to distinguish between silicon and carbon displacements by carrying out near-threshold electron irradiation experiments using a transmission electron microscope. The displacement energies for carbon and silicon are found, respectively, to be about 100 keV and 250 keV. After irradiation the samples have been studied by low-temperature photoluminescence spectroscopy. By carrying out the electron bombardment along (0001) and (0001) directions significant anisotropy of behaviour is found near the carbon displacement threshold. While the products of silicon displacement are found to be restricted to a region of irradiation those of carbon displacements are found to extend outside this region. Spectral features of higher photon energy than the laser used are explained by a two-photon absorption process.