Core luminescence as a method of studying the relaxation processes in the outermost core of crystals

Abstract

A specific kind of impurity emission emerging from radiative recombination of the valence electrons with highest-lying caesium core holes in mixed ACl–CsCl (A=Li, Na, K, Rb) systems is studied using pulse X-ray source and VUV synchrotron radiation for the excitation. The emission interpreted as impurity core luminescence (CL) is detected in the spectral region of about 4.5 eV in the compounds studied. The luminescence spectrum shape is distinctly determined by the caesium concentration, demonstrating the influence of the structure of the nearest anion environment of Cs. In agreement with the nature of the impurity CL, the excitation spectra of the studied materials exhibit clear onset in the region of photoexcitation of the 5p Cs impurity core states. Manifestation of the host core excitons resulting in apparent anticorrelation of the reflection (absorption) and emission yield spectra in the core exciton region is observed. The effect is supposed to originate from surface losses of the excitation energy and luminescence quenching caused by switching in the host cation core excitation. The decisive influence of the Auger decay of these core excitations suppressing the emission yield can account for the observed spectral dependence of the impurity CL excitation spectra.