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 samples 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.
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