On the luminescence, excitation and UV-absorption spectra of CsI activated with Na

The luminescence of CsI: Na powders and vapour-deposited layer has been studied. Various luminescence bands were found both at room temperature and at 77 K. One of them (the 'blue' band) is clearly related to the presence of Na. The UV excitation bands at 300 K and 77 K of this 'blue' luminescence have shapes which are strongly dependent on the Na-content and the sample thickness. Measurements show that the UV-absorption band giving rise to the blue luminescence is situated at wavelengths were host-lattice absorption also occurs. The measured absorption coefficients of the band due to the presence of Na and those of the host-lattice absorption allow a qualitative explanation of the various forms of the excitation spectra. Due to experimental limitations, a more quantitative approach is possible in a few cases only. The measured absorption strength of the Na band is proportional to the Na concentration in the 0.001-1 at.% range. The observed figures are about one order of magnitude lower than the estimated ones from the decay time of the blue luminescence by a theory of Dexter. This result suggests that the absorbing and the emitting state of the Na-centre are not identical. For this reason, no conclusion can be drawn as to whether all sodium atoms present in CsI give rise to the formation of luminescent centres