Structure of the self-interstitial in diamond

We report on a study of the structure of the neutral self-interstitial ${\mathrm{I}}^0$ 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 $5.0\pm 0.1\mathrm{meV}$ splitting, revealing the existence of another electronic state at ${\mathrm{I}}^0$ that has not been discussed within existing models of the center. The excited-state doublet couples through $B_2$ deformations, while the well-known ground-state doublet, whose splitting is measured spectroscopically at $7.6\pm 0.1\mathrm{meV},$ is coupled by $B_1$ deformations of the center. The data are quantitatively consistent with ${\mathrm{I}}^0,$ in its ground electronic state, tunneling rapidly in a $B_1$ vibrational mode between equivalent $D_2$-symmetry configurations, and in its excited electronic state tunneling in a $B_2$ mode between equivalent $C_{2v}$-symmetry configurations; in both cases, the motion is sufficiently rapid for ${\mathrm{I}}^0$ to have the observed effective $D_{2d}$ point group.