Self-interstitial in diamond

The self-interstitial in diamond produces optical absorption in two narrow lines at 1684.6 and 1859.4 meV. In C-13 diamond the 1685 meV line shifts by 1.7+/-0.2 meV to higher energy compared to natural diamond, while the 1859 meV line shifts by 5.35+/-0.25 meV to lower energy. The 1859-meV line increases in intensity at low temperature with an activation of 6.2 +/- 0.5 meV. These data are used to establish that the 1859-meV line originates in a forbidden electronic transition that is made allowed by mixing with the 1685 meV line through a vibrational mode with a quantum of 168.6 +/- 0.9 meV. All the available data are shown to be consistent with the (001) split interstitial relaxing from the ideal D-2d symmetry to a (static) D-2 configuration, plus a (dynamic) vibronically induced deformation of the center.