absolutely quantifying the concentration of a defect and is therefore fundamentally the source of the optical absorption cross-section values for most defects in diamond [@Davies1999]. Where charge transfer is present, the loss (gain) of the EPR-active charge state can be equated to the gain (loss) in the other, allowing an optical absorption cross-section to be extracted for the non-EPR-active state. The major assumption made in the above procedure is that there are only two charge states of the defect accessible through the charge transfer protocol, and therefore the loss of one must equal the gain in the other. If this is not the case, and some charge population is lost to a third charge state, then an incorrect optical absorption cross-section for the non-EPR-accessible charge state will be extracted.

Experimental detail
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Method and samples
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Seven samples were grown in a microwave-plasma chemical vapor deposition (CVD) reactor: Samples A – F were grown simultaneously and doped with natural abundance silicon (via the addition of silane to the growth gases) and [$^{15}\mathrm{N}$]{}-enriched nitrogen; [Sample G]{}was doped with natural abundance silicon and nitrogen (Table \[tab:sample\_summary\]). Growth substrates (all -oriented) and non-diamond material were removed from all samples post-growth to leave free-standing 