Photoionization processes in barium fluorohalide crystals doped with Eu²⁺

The absorption, excitation and photostimulation spectra and thermoluminescence creation spectra of pure BaFX (X = Br, Cl) and BaFX:Eu²⁺ single crystals were studied in the vacuum-ultraviolet (VUV) region 5-10 eV at temperatures of 6-300 K. Creation of a V_K thermoluminescence peak was observed when the BaFX:Eu²⁺ crystal was illuminated with VUV photons with energies above 6 eV while band-to-band transitions begin above 8-9 eV. Low-temperature photostimulation spectra for Eu²⁺ luminescence created by 6-7 eV photons show new bands in the infrared region, with intensities directly proportional to the Eu²⁺ concentration. However, the same infrared photostimulation bands for exciton luminescence were also observed for undoped crystals after x-ray irradiation. Therefore, a trace concentration of the shallow electron traps is present in each undoped and Eu-doped BaFBr and BaFCl crystal. The results show that self-trapped holes and electrons are trapped on different non-europium centres, created by VUV photons in BaFX:Eu²⁺ crystals at low temperature.

It was concluded that Eu²⁺ ions assist in the creation of free electrons and self-trapped holes by photons with an energy 2-3 eV lower than those of band-to-band transitions.