Exciton emission in BaFBr and BaFCl crystals

The emission of self-trapped excitons produced by vacuum ultraviolet excitation as well as X-ray excitation or photostimulation in BaFCl and BaFBr crystals was studied. The exciton emission bands at 4.2 and 5.15 eV in BaFBr and at 3.3 eV in BaFCl were observed upon irradiation into the Cl or Br exciton absorption bands. The 5.15 eV emission band in BaFBr thermally transforms to the 4.2 eV band with increasing temperature within the range 50-80 K. The 3.3 eV emission in BaFCl increases in the range 20-50 K in BaFCl, while the lifetime of the emission decreases. These facts indicate rather clearly the potential barrier between two different exciton configurations. The low-temperature exciton configuration was tentatively assigned to the Vk+e type and the other to F+H type.