The dependence of the luminescence of diamonds with negatively chargednitrogen-vacancy centers (NV-) vs. applied magnetic field (magnetic spectrum)was studied. A narrow line in zero magnetic field was discovered. Theproperties of this line are considerably different from those of other narrowmagnetic spectrum lines. Its magnitude is weakly dependent of the orientationof the single-crystal sample to the external magnetic field. This line is alsoobserved in a powdered sample. The shape of the line changes greatly whenexcitation light polarization is varied. The magnitude of the line has anon-linear relation to excitation light intensity. For low intensities thisdependence is close to a square law. To explain the mechanism giving rise tothis line in the magnetic spectrum, we suggest a model based on thedipole-dipole interaction between different NV- centers.
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