Creation of F centres and multiplication of electronic excitations in Na6Al6Si6O24(NaBr)2x optical ceramics under VUV irradiation

Abstract

The reflection spectra, the excitation spectra of various emissions (2.4–4.5 eV) and the creation spectra of F centres have been measured in Na6Al6Si6O24(NaBr)2x optical sodalite ceramics (x = 0.94 and x = 0.81) using VUV radiation of 6–35 eV. An analysis of the spectra allowed us to separate several groups of electronic excitations: the photons of 6.7–8.3 eV excite or ionize Br− centres in the β-cages, 8.5–32 eV photons generate electronic excitations of the aluminosilicate carcass, while 33 eV photons excite Na+ ions up to the 2p53s state. The creation mechanism of F centres, connected with the trapping of conduction electrons by pre-irradiation bromine vacancies and localization of holes at Br− centres in β-cages, has been revealed in sodalites at 80 K. The efficiency of F centre creation triples in the multiplication region of electronic excitations of the aluminosilicate carcass (20–30 eV) in a sample with x = 0.94. Using methods of thermoactivation spectroscopy, the creation of thermally stable F centres (up to 480 K) has been detected in sodalites irradiated by x-rays or 27 eV photons. However, such F centres cannot be created by 7.6–11 eV photons, i.e. when there is no multiplication of excitations and an exciting photon forms only one electronic excitation.