The symmetry properties of the ND1 absorption centre in electron- irradiated diamond

The ND1 centre is produced in natural and synthetic diamond by primary radiation damage, but only occurs in strength when substantial concentrations of nitrogen are present in the lattice. The optical absorption associated with this centre consists of a sharp no-phonon line at 3.149 ev (80 degrees K) and phonon-assisted structure at higher energies centred at about 3.4 ev. Uniaxial stress measurements on the no-phonon line have been interpreted in terms of an A to T transition at a centre of T_d symmetry, and from this and other evidence of the ND1 centre has been tentatively identified with an interstitial nitrogen atom.