Photoionization of negatively charged NV centers in diamond: Theory and ab initio calculations

We present ab initio calculations of photoionization thresholds and cross sections of the negatively charged nitrogen-vacancy (NV) center in diamond. We consider photoionization from the ground A23 and the excited E3 states. After the ionization from the E3 level, we show that the NV center transitions into the metastable A24 electronic state of the neutral defect. We reveal how spin polarization of NV- gives rise to spin polarization of the A24 state, explaining electron spin resonance experiments. We obtain smooth curves of photoionization cross sections as a function of energy by employing dense k-point meshes for the Brillouin-zone integration together with the band unfolding technique to rectify the distortions of the band structure induced by the artificial periodicity of the supercell approach. Our calculations provide a comprehensive picture of photoionization mechanisms of NV-. They will be useful in interpreting and designing experiments on charge-state dynamics at NV centers. In particular, we offer a consistent explanation of recent results of spin-to-charge conversion of NV centers.