The electronic structure of the tetrachedral N₂ centre and of the neutral vacancy in diamond

The N₂ centre, a tetrachedral array of four F centres, and the neutral vacancy in diamond present similar theoretical problems. Their electronic structure has been calculated by a molecular orbital approach using one-electron wave functions centred on the geometric centre of the defect. The results are compared with those based on functions which are linear combinations of atomic orbitals (LCAO). We conclude that in LiF and KC1 the ground state of the N₂ centre is ⁵A₂ and the centre is not responsible for the N₂ band, although it should give rise to spin resonance. For the diamond vacancy the ground state is ³T₁ or possibly ¹E. Previous calculations using LCAO wave functions give a ¹E ground state for diamond. Possible ways of observing these centres are discussed.