Electron-hole and exciton processes in CaF$_2$, SrF$_2$, and BaF$_2$ crystals (Review)

The main characteristics of nominally pure CaF$_2$, SrF$_2$, and BaF$_2$ single crystals are presented. Numerous experimental and theoretical data on the crystal and band structures of the compounds under study and their optical properties have been brought together; the analysis and systematization of these data were carried out. It is shown that many features of CaF$_2$, SrF$_2$, and BaF$_2$ are due to their crystal structure. The main parameters of electron (F, M) and hole (V$_K$, H) point defects are presented, and the mechanisms of their diffusion at low and high temperatures are studied. The transient absorption spectra of defects are explored. The criteria for various channels of defect formation and presumable mechanisms for the decay of self-trapped excitons with the production of point defects and intrinsic luminescence quanta are analyzed. The problem of interpretation of the emission mechanism of singlet excitons is considered.