The effect of radiation damage and annealing upon the optical properties of N-gettered nearly-IIa-type synthetic diamonds was studied. It was found that the 2.156eV centre, whose absorption was usually very weak, was here one of the predominant absorption features. A new vacancy-related vibronic absorption system, with zero-phonon line at 3.420eV, was observed. A pronounced photochromic effect was established for the 1.945, 2.085, 2.156, 3.420 and 4.325eV absorption bands. Of these, only the 2.156eV band remained in the absorption spectra after photo-excitation. In similarly treated type-Ib diamonds, there were no photo-induced changes in the 1.945eV band absorption. On the basis of the results of optical bleaching and thermal recovery experiments, it was concluded that the attribution of the 2.156eV centre to the neutral charge state of the N-vacancy defect needed further consideration.

Photochromic Effect in Irradiated and Annealed Nearly IIa-Type Synthetic Diamond. I.N.Kupriyanov, V.A.Gusev, Y.N.Palyanov, Y.M.Borzdov: Journal of Physics - Condensed Matter, 2000, 12[35], 7843-56