The use of ENDOR to identify the atomic structure of defects in diamond

Although nearly 100 paramagnetic defects have been catalogued in diamond by spin Hamiltonian parameters measured by electron paramagnetic resonance (EPR), very few of these have been unambiguously associated with an atomic model. It has been necessary to use electron nuclear double resonance (ENDOR) to obtain enough information to make proper assignment of such models. The reason for the limitation of EPR, and the way in which ENDOR overcomes these limitations are discussed. The interpretation of hyperfine structure in terms of unpaired electrons in molecular orbitals, and of quadrupole interactions in terms of all electrons, paired and unpaired, as a source of information about molecular structure in diamond, is evaluated by reference to some well documented examples. The measurements so far made by ENDOR on defects in diamond are reviewed, and the salient contribution for the assignment of a model for each defect is explained. The details revealed by ENDOR considerably increase knowledge about defects, particularly those involving substitutional nitrogen atoms. This in turn helps in understanding the complex electron and atom, migration processes which go on under appropriate conditions of temperature and pressure, or optical excitation. The possibilities are discussed for using ENDOR to increase the number of well characterized centres.