Electron nuclear double resonance of the F centre in CaF2

Although a considerable body of work exists on colour centres in the alkaline earth fluorides, precise information on the structure of intrinsic point defects in these crystals is meagre. Arends (1964) reported an e. p. r. spectrum in additively coloured CaF2 which he assigned to the F centre. We have carried out endor measurements on additively coloured CaF2 which confirm that the e. p. r. spectrum described by Arends is indeed due to the F centre. We have measured the hyperfine interaction of the F centre electron with the nuclei of the first four fluorine shells centred on the F centre and we compare our results with preliminary calcula­tions of the hyperfine constants by A. M. Stoneham (private communication). The effects of fluorine-fluorine interaction on the endor spectrum of the first shell fluorines are investigated in detail.