A study was made of the structure of the neutral self-interstitial, I0, via uni-axial stress measurements and isotope-substitution studies of the optical absorption lines near to 1.685 and 1.859eV. The stress perturbations were explained in terms of a center having D2d symmetry, and the predominant stress-induced perturbations were found to involve interactions between the states of the center. The inter-state couplings indicated that the excited electronic state of the transitions was a doublet, with 0.005eV splitting; thus revealing the existence of another electronic state at I0 which had not been considered by existing models for the center. The excited-state doublet coupled via B2 deformations, while the familiar ground-state doublet (whose splitting was measured spectroscopically as 0.0076eV) was coupled by B1 deformations of the center. The data were quantitatively consistent with I0, in its ground electronic state, tunneling rapidly in a B1 vibrational mode between equivalent D2-symmetry configurations and, in its excited electronic state, tunneling in a B2 mode between equivalent C2v-symmetry configurations. In both cases, the motion was sufficiently rapid for I0 to have the observed effective D2d point group.

Structure of the Self-Interstitial in Diamond. H.E.Smith, G.Davies, M.E.Newton, H.Kanda: Physical Review B, 2004, 69[4], 045203 (9pp)