  The positively charged nitrogen vacancy (NV+) centre in diamond has been
traditionally treated as a dark state due to the experimental lack of an
optical signature. Recent computational studies have shown that it is possible
for the NV+ defect to have an excited state transition equivalent to that of
the negatively charged (NV-) centre, but no PL predictions have been reported
so far. We report the first ab-initio calculation showing that the NV+ center
presents quantum emission, with zero phonon line at 765 nm and a non-zero
transition dipole moment, approximately 4x smaller than the transition dipole
moment of NV-. We calculate the energy levels of the multielectron states under
time-dependent density functional theory (singlet and triplet E states), and
using our recently developed frequency cutoff method, we predict the full PL
spectrum. Our results suggest that this state cannot be considered
intrinsically 'dark' and charge specific quenching mechanisms should be
investigated as the cause of the lack of optical activity in experimental
characterizations.
