Hyperfine interaction of defects in insulators

The study of point defects, other than impurities, in insulators by electron paramagnetic resonance (E.P.R.) and electron nuclear double resonance (E.N.D.O.R.) is discussed. An introduction to the theory of the hyperfine spectra as obtained by E.P.R. and E.N.D.O.R. is presented in order to provide a basis for the discussion of results and their interpretation. Emphasis is placed on how the theory is used to analyse the spectra and how characteristics of the defects such as electronic state and defect geometry are obtained from the analyses. A résumé of the experimental techniques, especially E.N.D.O.R., is presented to give the reader a reasonably current view of the methods for obtaining data. The presentation of experimental results and recent theoretical developments is divided into a discussion of electron excess defects in cubic lattices and one on trapped hole centres in the same lattice type. This is followed by a discussion of these types of defects in non-cubic lattices. Finally the results of measurements on the hyperfine structure of excited states of electron and hole type in cubic crystals are discussed. In all cases an attempt was made to present the details of an illustrative example in order to indicate the salient features of the topic rather than give an exhaustive discussion. Needs for further measurement and future improvements of the theory are indicated.