Cathodoluminescence studies of isotope shifts associated with localised vibrational modes in synthetic diamond

Sharp structure between 2.60 and 2.64 eV observed in the cathodoluminescence spectra of synthetic diamond, following radiation damage and annealing at 300 degrees C, has been identified as a series of local-mode phonon replicas of the zero-phonon line at 2.807 eV. By examining the isotope shifts of these replicas in diamonds grown using 13C (and containing natural nitrogen) compared with diamonds synthesised from natural carbon and doped with 15N, the authors conclude that the optical centre contains one nitrogen atom in association with the interstitial produced by the radiation damage. The local-mode structure is very similar to that observed for the system with a zero-phonon line at 3.188 eV, which has also been shown to be a single nitrogen plus interstitial centre. Coupling of the 2.807 eV electronic transition to lattice modes is, however, much weaker than for the 3.188 eV transition. In diamonds which have not been thermally annealed the intensities of these vibronic bands, initially zero, are observed to increase with increasing time under the influence of the electron beam-a phenomenon observed previously for the 2.156 eV optical centre. The absence of localised vibrational modes, the response of the zero-phonon line to random strain and the greatly increased luminescence intensity following annealing at 800 degrees C lead us to propose that the 2.156 eV centre involves single nitrogen with one or more vacancies.