The unique optical properties of the nitrogen-vacancy (NV) center in diamond have established it as a workhorse for quantum science applications. Both charge and spin dynamics, driven by laser excitation, are critically mediated by the singlet state, yet several key mechanisms remain elusive. In this work, we provide experimental evidence of a novel charge conversion pathway, a transition from the neutral charge state (${\mathrm{NV}}^0$) to the singlet manifold of the negative charge state (${\mathrm{NV}}^{-}$) under high laser power. Furthermore, we directly confirm the two-photon ionization nature of the singlet state using near-infrared excitation and present an extensive discussion of the ionization mechanisms of ${\mathrm{NV}}^{-}$. These findings offer deeper insights into the singlet-mediated charge-spin interplay, providing a foundation for potential advancements in spin readout technologies of NV centers.

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• NV centers
• Quantum optics
• Nitrogen vacancy centers in diamond
• Electron spin resonance
• Optical pumping
• Photoionization
• Time-resolved photoluminescence