Absorption, Excitation, and Fluorescence Spectra of Thallium-Activated Ammonium Chloride

Abstract

Absorption and fluorescence spectra of ammonium chloride activated by varied amount of thallium have been investigated at various temperatures in order to study the nature of electronic states involved in the luminescence centres. Absorption bands at 6.05, 5.84, 5.63, 5.38, 5.17, 5.00, and 4.86 eV and emission bands at 3.45, 2.70, and 2.30 eV have been observed. The disappearance of visible bands, the shift of the ultraviolet band from 3.45 to 3.65 eV, and the shift of the A absorption band (1A1g → 3T1u) towards longer wavelengths at about the transition temperature (184°C) have been correlated to the change in the crystal structure of the host lattice from b.c.c. to f.c.c. The excitation and emission spectra have been recorded at liquid nitrogen temperature to ascertain the electronic transitions of different emission bands. The absorption and fluorescence bands have been attributed to the electronic transitions in monomer and dimer centres having cubic and tetragonal site symmetries.