Photochromic effect in irradiated and annealed nearly IIa type synthetic diamond

We examined the effect of radiation damage and annealing on the optical properties of nitrogen-gettered nearly IIa type synthetic diamonds. It was found that the 2.156 eV centre, whose absorption is usually very weak, appears in these diamonds as one of the dominant absorption features. A new vacancy-related vibronic absorption system with zero-phonon line at 3.420 eV was observed. A pronounced photochromic effect was established for the 1.945, 2.085, 2.156, 3.420 and 4.325 eV absorption bands. Of all these bands only the 2.156 eV band remains in the absorption spectra after a proper photoexcitation. In contrast, for similarly treated type Ib diamonds we did not reveal any photoinduced changes in the 1.945 eV band absorption. Based on the results of optical bleaching and thermal recovery experiments, we conclude that the assignment of the 2.156 eV centre to the neutral charge state of the nitrogen-vacancy defect needs further verification.