Infra-red (IR) absorption results on irradiated and annealed synthetic diamond are presented which confirm an earlier proposal that a component found in the defect-induced one-phonon region of some diamonds arises from positively charged single-substitutional nitrogen . The concentration ratio of to neutral substitutional nitrogen centres may be changed by shining light of various energies onto the examined samples. By correlating changes in absorption of the IR component associated with centres with changes in the component, and using a previously determined relation between the concentration of centres and peak absorption coefficient at 1130 , the relationship between peak absorption at 1332 and concentration of centres has been derived, namely 1 of absorption is produced by ppm centres. Other defects may also give rise to absorption at 1332 , but the component is uniquely identified by further peaks at 1046 and 950 . The significance of this component is demonstrated by the fact that some samples can contain in excess of 80 ppm centres, and this must consequently be accounted for when assaying the total nitrogen concentration in such samples. Using the above relationship useful parameters relating the concentration of neutral vacancies, negative vacancies and negatively charged nitrogen-vacancy centres to their respective zero-phonon line integrated absorptions have been derived.

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