Measurements of the electron paramagnetic resonance during photo-excitation were performed for Ni defects in diamonds grown by using a Ni-containing solvent or catalyst. The temperature-dependence of the W8 electron paramagnetic resonance spectrum photo-quenching showed that the relaxation of substitutional Nis- upon electron ionization was very small; thus supporting the interpretation that previously reported photo-induced effects, with thresholds at 2.5 and 3.0eV, corresponded to 2 complementary photo-ionization transitions involving Nis. The photo-induced behaviour of the NIRIM1 electron paramagnetic resonance center favored the interstitial Nii+ model for this defect and suggested that the Nii0/+ level was located at 1.98eV below the conduction band. In N-doped diamond, Nii was more likely to appear in the neutral state (undetectable by electron paramagnetic resonance), whereas Nis- was revealed at substitutional sites.

Observation of a strong AB2 electron paramagnetic resonance signal photo-quenching, and the simultaneous detection of different spectral dependences of the electron paramagnetic resonance intensity for other defects, indicated an electron photo-ionization energy of 1.67eV for AB2. The study showed that Ni defects exhibited a weak electron-lattice interaction. The importance of the stronger spin-orbit coupling in these centers, as compared with other defects, was pointed out.

Photoexcitation Electron Paramagnetic Resonance Studies on Nickel-Related Defects in Diamond. R.N.Pereira, W.Gehlhoff, A.J.Neves, N.A.Sobolev: Journal of Physics - Condensed Matter, 2003, 15[16], 2493-505