the sample in the two extremal charge states.[]{data-label="fig:UV-vis-ch"}](graphs_combined_uv-vis-ch-transfer_w_images){width="\columnwidth"}

Discussion
----------

Doubly-charged defects are well-characterized in other group-IV semiconductors (e.g. Si [@Watkins2000; @Newman1982] and Ge [@Weber2013; @Weber2015]) but have not been previously reported in diamond despite several theoretical predictions [@Breuer1995; @Mainwood1997; @Thiering2015]. Generally, this is a result of the paucity of very shallow donors and acceptor states in diamond, which are required to stabilize the chemical potential suitably for these doubly-charged states (in the absence of negative-U effects [@Watkins1980]). However, in [$\mathrm{SiV}$]{}the (2$-$/$-$) transition is relatively deep (approximately mid-gap [@Gali2013]), yielding a stable charge state even for deep nitrogen donors (at approximately $E_C-\SI{1.7}{\electronvolt}$ [@Farrer1969a]).

Despite its lack of internal transitions, we still expect transitions from the [$\mathrm{SiV}^{2-}$]{} ground state to the conduction band, which are theoretically predicted at [@Gali2013]. As a defect-to-band transition, this will manifest as spectrally broad rather than a sharp transition. There is a small change in the absorption gradient () between the UV and heated states, but any absorption in this region is dominated by [$\mathrm{N_{s}^{0}}$]{} absorption \[Fig. \[fig:UV-vis-ch\]\] [@Jones2009b] and hence difficult to isolate.

In [Sample D]{}we measure the UV state concentrations of [$\mathrm{SiV}^{-}$]{} and [$\mathrm{SiV}^{0}$]{} as and , respectively \[Fig \[fig:UV-vis-ch\]\], using the conversion factors 