  We introduce a microwave-assisted spectroscopy technique to determine the
relative concentrations of nitrogen vacancy (NV) centers in diamond that are
negatively-charged (NV${}^-$) and neutrally-charged (NV${}^0$), and present its
application to studying spin-dependent ionization in NV ensembles and enhancing
NV-magnetometer sensitivity. Our technique is based on selectively modulating
the NV${}^-$ fluorescence with a spin-state-resonant microwave drive to
isolate, in-situ, the spectral shape of the NV${}^-$ and NV${}^0$ contributions
to an NV-ensemble sample's fluorescence. As well as serving as a reliable means
to characterize charge state ratio, the method can be used as a tool to study
spin-dependent ionization in NV ensembles. As an example, we applied the
microwave technique to a high-NV-density diamond sample and found evidence for
a new spin-dependent ionization pathway, which we present here alongside a
rate-equation model of the data. We further show that our method can be used to
enhance the contrast of optically-detected magnetic resonance (ODMR) on NV
ensembles and may lead to significant sensitivity gains in NV magnetometers
dominated by technical noise sources, especially where the NV${}^0$ population
is large. With the high-NV-density diamond sample investigated here, we
demonstrate up to a 4.8-fold enhancement in ODMR contrast. The techniques
presented here may also be applied to other solid-state defects whose
fluorescence can be selectively modulated by means of a microwave drive. We
demonstrate this utility by applying our method to isolate room-temperature
spectral signatures of the V2-type silicon vacancy from an ensemble of V1 and
V2 silicon vacancies in 4H silicon carbide.
