Journal of the American Ceramic Society, Vol.103, No.10, 5480-5487, 2020
Vibronic spectra of europium in the X-ray storage phosphor polycrystalline cesium bromide
Cesium bromide doped with europium is a second-generation X-ray storage phosphor used in radiographic imaging. X-irradiation generates free electrons and holes, some of which are trapped at lattice defects and impurities. Irradiation with visible (red) light frees the electrons from their traps and they can recombine with the trapped holes, with the energy of recombination appearing as blue luminescence from the europium ion. Although the electron trap is widely held to be a bromine vacancy, the nature of the hole trap is as yet undetermined. Here we report on the remarkable sharp-line photoluminescent emission of Eu2+ ions which appears below 50 K in activated europium-doped cesium bromide and comprises over 20 individually resolvable lines. These patterns of vibronics may give some insight into the nature of the hole trap. A detailed analysis shows that the vibronics comprise five separate patterns, but with a common vibrational frequency of similar to 123 cm(-1). The vibronic patterns are attributed to Br-2(-) molecular ions adjacent to the europium ion. An interpretation of the observed spectra in terms of a Eu2+-Eu3+ ion pair charge compensated by two cation vacancies and a substitutional O2- ion is discussed.