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© 2016 American Physical Society. We present an experimental and theoretical study of the optically detected magnetic resonance signals for ensembles of negatively charged nitrogen-vacancy (NV) centers in a C13 isotopically enriched single-crystal diamond. We observe four broad transition peaks with superimposed sharp features at zero magnetic field and study their dependence on an applied magnetic field. A theoretical model that reproduces all qualitative features of these spectra is developed. Understanding the magnetic-resonance spectra of NV centers in an isotopically enriched diamond is important for emerging applications in nuclear magnetic resonance.

We present an experimental and theoretical study of the optically detected magnetic resonance signals for ensembles of negatively charged nitrogen-vacancy (NV) centers in a C13 isotopically enriched single-crystal diamond. We observe four broad transition peaks with superimposed sharp features at zero magnetic field and study their dependence on an applied magnetic field. A theoretical model that reproduces all qualitative features of these spectra is developed. Understanding the magnetic-resonance spectra of NV centers in an isotopically enriched diamond is important for emerging applications in nuclear magnetic resonance.

It is the hyperfine structure of N-14 and C-13 in the electron paramagnetic resonance (EPR) spectrum which indicates that the unpaired electron of a single substitutional nitrogen atom in diamond is in one of the four anti-bonding N-C orbitals. We show that, for diamonds containing a very high concentration of nitrogen, the hyperfine structure of interacting pairs of nitrogen atoms indicates that for close neighbours there are unique orientations of the constituent N-C bonds, while at larger distances the orientations are random.

Results are reported of both isochronal and isothermal annealing studies of the R1 EPR centre (known to be a pair of parallel nearest-neighbouring -split self-interstitials) produced by 2 MeV electron irradiation of synthetic type IIa diamonds of very low defect concentration before irradiation. The annealing out of R1 is not associated with any overall change in the concentration of isolated vacancies or interstitials. The isothermal decay corresponds to a first-order process with a rate constant conforming to an Arrhenius relationship with activation energy 0.6(1) eV and an unusually low attempt frequency. That the activation energy is much lower than the migration energy of isolated interstitials (1.68(15) eV) is taken to indicate that it corresponds to a combination of the energy required to excite R1 into an S = 0 state and that required to translate one of the split interstitials over the potential barrier into the next lattice site. This process may be linked to an associated rise in the concentration of the 3H (2.462 eV) optical defect; but the precise relationship between the diamagnetic site formed by the decay of R1 and the 3H optical centre is not revealed by this study.

New measurements on the R1 intrinsic radiation damage centre in diamond, recently identified as a di-[100]-split interstitial (DSI), indicate that (a) it can be spin polarised by radiation of energy greater than 1.7 eV, and (b) it is destroyed by annealing at about 600K with an associated transfer of an electron to a neutral vacancy, producing a temporary increase in the density of negatively charged vacancies.