Self-interstitial in diamond

The self-interstitial in diamond produces optical absorption in two narrow lines at 1684.6 and 1859.4 meV. In ${}^{13}\mathrm{C}$ diamond the 1685 meV line shifts by $1.7\pm 0.2\mathrm{meV}$ to higher energy compared to natural diamond, while the 1859 meV line shifts by $5.35\pm 0.25\mathrm{meV}$ to lower energy. The 1859-meV line increases in intensity at low temperature with an activation of $6.2\pm 0.5\mathrm{meV}.$ 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 $168.6\pm 0.9\mathrm{meV}.$ 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.