Together with [$\mathrm{SiVN}^0$]{} this corresponds to a total of nitrogen, accounting for the majority of the lost [$\mathrm{N_{s}^{0}}$]{}$^{/+}$ and [${\ensuremath{\mathrm{NVH}}\xspace}^{0}$]{}$^{-}$. However, a significant concentration of nitrogen-related defects remain unidentified. As [$\mathrm{N_3VH}$]{} contains three nitrogen atoms, a small error in its oscillator strength would have a dramatic effect on our ability to quantify total nitrogen in the high-temperature annealed samples.

![a) IR absorption spectra of [Sample A]{}(top) and [Sample F]{}(bottom) in the two extremal charge states. The two regions give the defect-induced one-phonon absorption (left) and C-H stretch region (right): the intrinsic multiphonon absorption has been subtracted. b) The one-phonon of two as-grown samples grown under similar CVD conditions: both are nitrogen-doped but silicon was added to the growth gasses of one. The primary difference is the feature at approx.  in the silicon-containing sample, which is tentatively assigned to substitutional silicon [@Goss2007]. The remaining peaks are reported in studies of solely nitrogen-doped CVD material [@Khan2009]. c) Difference spectra between [Sample A]{}(as-grown) and samples treated at the given temperatures. The change in the mode is highlighted.[]{data-label="fig:ir_comparison"}](graphs_combined_ir_comparisons_with_si_highlight){width="\columnwidth"}

Photochromism and evidence for SiV2-
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In previous studies of nitrogen-doped brown CVD diamond, samples which were annealed above became less brown, with higher temperatures corresponding 