Misidentification of nitrogen-vacancy absorption in diamond

The annealing of electron-irradiated type IaB diamond at 1600degreesC substantially reduces the concentration of H4 centres, and increases the concentration of H3 centres. In addition, defects are created producing absorption in zero-phonon lines near 536 and 575 nm. Occasionally these same absorption lines are seen at considerable strength in natural brown type la diamonds. The interpretation of the annealing behaviour proposes that the H4 centre (four nitrogen atoms and two vacancies) is dissociating to produce smaller defects. The H3 centre, for example, contains two nitrogen atoms and a vacancy, and it has been assumed that the absorption line close to 575 nm is the zero-phonon transition at the neutral nitrogen-vacancy centre. In this work we show that the line, and the line seen for natural brown diamonds, is an independent transition at an unknown defect centre.