Vibrational properties and photoionization of color centers in diamond: theory and ab initio calculations

Over the past two decades, optically active deep-level point defects in semiconductors became leading building blocks for quantum technologies such as quantum computing, quantum-enhanced sensing, and quantum communication. This thesis addresses several theoretical issues in the physics of such defects. The thesis presents new first-principles theoretical methods to investigate the vibrational and vibronic structure of deep-level defects. These methods are then used to explain the vibrational structure of negatively charged silicon-vacancy (SiV-) and nitrogen-vacancy (NV-) centers in diamond. Another part of the dissertation deals with the modeling of optical spectra, the determination of which should serve for the analysis and identification of new defects. We advance the ab initio methodology to describe electron-phonon interactions and apply it to describe optical spectra of the diamond NV- center. In the final part of the thesis, we investigate the photoionization mechanisms of the NV- center, during which the defect changes its charge state to a neutral one. A new methodology to smooth absolute photoionization cross-sections is presented and applied to explain recent photoionization experiments and charge state dynamics.