Structure of the self-interstitial in diamond

We report on a study of the structure of the neutral self-interstitial I⁰ in diamond, through the use of uniaxial stress measurements and isotope-substitution effects on the optical absorption lines near 1685 and 1859 meV. The stress perturbations are explicable in terms of a center with D_2d symmetry, and the dominant stress-induced perturbations are found to be interactions between the states of the center. The interstate couplings establish that the excited electronic state of the transitions is a doublet, of 5.0±0.1 meV splitting, revealing the existence of another electronic state at I⁰ that has not been discussed within existing models of the center. The excited-state doublet couples through B₂ deformations, while the well-known ground-state doublet, whose splitting is measured spectroscopically at 7.6±0.1 meV, is coupled by B₁ deformations of the center. The data are 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 C_2v-symmetry configurations; in both cases, the motion is sufficiently rapid for I⁰ to have the observed effective D_2d point group.