The annealing of electron-irradiated type-IaB diamond at 1600C substantially reduced the concentration of H4 centers, and increased the concentration of H3 centers. In addition, defects were created which produced absorption in zero-phonon lines near to 536 and 575nm. These same absorption lines were occasionally seen with considerable strength in

natural brown type-Ia diamonds. The interpretation of the annealing behaviour proposed that the H4 center (four N atoms and 2 vacancies) dissociated to produce smaller defects. The H3 center, for example, contains two N atoms and a vacancy, and it was assumed that the absorption line close to 575nm was the zero-phonon transition at the neutral N-vacancy center. It was shown that the line, and the line seen for natural brown diamonds, was an independent transition at an unknown defect center.

Misidentification of Nitrogen-Vacancy Absorption in Diamond. A.T.Collins, C.H.Ly: Journal of Physics - Condensed Matter, 2002, 14[25], L467-71