Photoionization processes in barium fluorohalide crystals doped with EU2+

The absorption, excitation and photostimulation spectra and thermoluminesce nce creation spectra of pure BaFX (X = Br, Cl) and BaFX:Eu2+ single crystal s were studied in the vacuum-ultraviolet (VUV) region 5-10 eV at temperatur es of 6-300 K. Creation of a VK thermoluminescence peak was observed when t he 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 phot ostimulation spectra for Eu2+ luminescence created by 6-7 eV photons show n ew bands in the infrared region, with intensities directly proportional to the Eu2+ concentration. However, the same infrared photostimulation bands f or exciton luminescence were also observed for undoped crystals after x-ray irradiation. Therefore. a trace concentration of the shallow electron trap s is present in each undoped and Eu-doped BaFBr and BaFCl crystal. The resu lts show that self-trapped holes and electrons are trapped on different non -europium centres, created by VUV photons in BaFX:Eu2+ crystals at low temp erature. It was concluded that Eu2+ ions assist in the creation of free electrons an d self-trapped holes by photons with an energy 2-3 eV lower than those of b and-to-band transitions.