Infra-red absorption data on irradiated and annealed synthetic diamonds were presented which confirmed the suggestion that a component which was found in the defect-induced 1-phonon region of some diamonds arose from single-substitutional N+. The concentration ratio of N+ to neutral substitutional N0 centers could be changed by shining light with various energies onto the samples. Changes in absorption, of the infra-red component which was associated with N0 centers, were correlated with changes in the N+ component. By using a previously determined relationship, between the concentration of N0 centers and the peak absorption coefficient at 1130/cm, a relationship was derived between the peak absorption at 1332/cm and the concentration of N+ centers. This relationship was that 1/cm of absorption was produced by 5.5ppm of N+ centers. Other defects could also give rise to absorption at 1332/cm, but the N+ component was uniquely identified by further peaks at 1046 and 950/cm. The significance of this component was illustrated by the fact that some samples could contain more than 80ppm of N+ centers, and this therefore had to be allowed for when estimating the total N concentration. By using the above relationship, useful parameters were derived which related the concentration of neutral vacancies, negative vacancies and negatively-charged N-vacancy centers to their respective zero-phonon line integrated absorptions.

S.C.Lawson, D.Fisher, D.C.Hunt, M.E.Newton: Journal of Physics - Condensed Matter, 1998, 10[27], 6171-80