Optical excitation of deep defect levels in insulators within many-body perturbation theory:: The F center in calcium fluoride

As a prototype for defects in insulators, we discuss the optical properties of the F center in calcium fluoride (CaF2), which constitutes a prominent defect of the material. The F center of CaF2 exhibits a defect state deep in the band gap. Its excitation by light (at 3.3 eV excitation energy) is described by ab initio many-body perturbation theory (GW approximation and the Bethe-Salpeter equation), including electronic exchange, correlation, and electron-hole interaction effects. The excitation causes strong relaxation of the defect geometry, leading to significant broadening of the optical spectrum by 0.5 eV and to a large Stokes shift of 1.5 eV.