It was recalled that the self-interstitial produced optical absorption in 2 narrow lines, at 1.6846 and 1.8594eV. In 13C diamond, the 1.685eV line shifted - by 0.0017eV - to higher energies as compared to that for natural diamond. Meanwhile, the 1.859eV line shifted - by 0.00535eV - to lower energies. The 1.859eV line increased in intensity at low temperatures, with an activation energy of 0.0062eV. The data were used to establish that the 1.859eV line originated from a forbidden electronic transition that was made possible by mixing with the 1.685eV line via a vibrational mode with a quantum of 0.1686eV. All of the data were consistent with a process in which the <001> split interstitial relaxed from the ideal D2d symmetry to a (static) D2 configuration; together with a (dynamic) vibronically-induced deformation of the center.

Self-Interstitial in Diamond. G.Davies, H.Smith, H.Kanda: Physical Review B, 2000, 62[3], 1528-31