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With the creation of nitrogen (NV) in 1b diamond it is common to find that the absorption and emission is predominantly of negatively charged NV centres. This occurs because electrons tunnel from the substitutional nitrogen atoms to NV to form NV ^− –N ^+ pairs. There can be a small percentage of neutral charge NV ^0 centres and a linear increase of this percentage can be obtained with optical intensity. Subsequent to excitation it is found that the line width of the NV ^− zero-phonon has been altered. The alteration arises from a change of the distribution of N ^+ ions and a modification of the average electric field at the NV ^− sites. The consequence is a change to the Stark shifts and splittings giving the change of the zero-phonon line (ZPL) width. Exciting the NV ^− centres enhances the density of close N ^+ ions and there is a broadening of the ZPL. Alternatively exciting and ionizing N ^0 in the lattice results in more distant distribution of N ^+ ions and a narrowing of the ZPL. The competition between NV ^− and N ^0 excitation results in a significant dependence on excitation wavelength and there is also a dependence on the concentration of the NV ^− and N ^0 in the samples. The present investigation involves extensive use of low temperature optical spectroscopy to monitor changes to the absorption and emission spectra particularly the widths of the ZPL. The studies lead to a good understanding of the properties of the NV ^− –N ^+ pairs in diamond. There is a critical dependence on pair separation. When the NV ^− –N ^+ pair separation is large the properties are as for single sites and a high degree of optically induced spin polarization is attainable. When the separation decreases the emission is reduced, the lifetime shortened and the spin polarization downgraded. With separations of