transfer procedure are not capable of destroying or creating [$\mathrm{SiV}$]{}and we conclude that we are efficiently driving to a third charge state of [$\mathrm{SiV}$]{}. The photochromic behavior of the samples is consistent with driving to a negatively- (rather than positively-) charged state. DFT studies of [$\mathrm{SiV}$]{}predict that [$\mathrm{SiV}^{2-}$]{}is a stable and electronically saturated system with no internal optical transitions or accessible spin levels and is thus difficult to spectroscopically observe [@Gali2013]. We therefore infer the presence of [$\mathrm{SiV}^{2-}$]{} by the absence of [$\mathrm{SiV}^{-}$]{} and [$\mathrm{SiV}^{0}$]{} in the heated charge state.

![UV-Vis absorption spectra of [Sample D]{}(annealed at ) measured at in two extremal charge states — see text for details. Strong [$\mathrm{SiV}^{-}$]{} and [$\mathrm{SiV}^{0}$]{} spectra are recorded in the UV-treated charge state, and are undetectable in the heated charge state. The charge transfer processes are reversible i.e. no net [$\mathrm{SiV}$]{} is created or destroyed in during the treatments: the dramatic loss of [$\mathrm{SiV}^{-}$]{} and [$\mathrm{SiV}^{0}$]{} between extremal states is therefore strong evidence for the existence of a third charge state of [$\mathrm{SiV}$]{}, which we identify as [$\mathrm{SiV}^{2-}$]{}. Note that a broad [$\mathrm{SiV}^{0}$]{} absorption continues beneath the [$\mathrm{SiV}^{-}$]{} absorption, but the structure between belongs to [$\mathrm{SiV}^{-}$]{}. Inset: representative transmission images of 