Luminescence of impurity-activated alkali halides under high pressure

The effects of high pressure on the luminescence from Tl -, In -, and Ga+ -doped alkali halide phosphors excited in the A-absorption band were studied at room temperature for a number of materials in both the fcc and sc crystal structures. An emission spectrometer was constructed for use in the visible and near UV. The luminescent materials were investigated to 140 kbar in quasihydrostatic high pressure optical emission cells. Using a model for the pressure effects on a simple configuration coordinate description of this localized center, an interpretation of the luminescence and absorption spectra is given. Froln the high pressure studies, it is clear that the single configuration coordinate description is inadequate. The state to which the electron is excited in the absorption process is not, in general, the same electronic state from which emission occurs; the behavior with pressure of the optical absorption and emission transitions are not consistent with the single configuration coordinate approach. However, each process (luminescence or absorption) can be treated separately in terms of such a model, and the various parameters calculated which describe the configuration coordinate model. In particular, several methods are presented which give consistent results for R = (w'/w)2, the ratio of the excited and ground electronic state force constants, as a function of pressure for the emission from the high-pressure (sc) phase of KI:Tl-type phosphors. The emission from KI:Tl-type phosphors is known to display a doublet structure, due to both Jahn-Teller interactions and intermediate spin-orbit coupling. The effect of pressure on the room temperature emission is to cause a redistribution of intensity between the peaks of the doublet, with higher pressure favoring the higher energy A.r emission, known to occur from an excited state of tetragonal symmetry. In the high pressure CsCI-phase, a similar emission doublet was observed in both In+-and TI+-doped potassium and cesium halides. Again, pressure causes a redistribution of the intensity within the doublet spectrum. A model was developed which describes these relative intensity changes in terms of the relative shifts with pressure of the emission peak energies. The mechanism for the relative energy shifts and intensity changes appears to be due to the increase with pressure of the electron-lattice coupling parameter for interaction modes of tetragonal symmetry. Three emission bands were observed in CsI:TI at different pressures. It is suggested that these emissions occur from an excited state which is Jahn-Teller-split into states of trigonal, rhombic, and tetragonal symmetry. An example of an electron transfer process was investigated by considering photostimulated luminescence in NaCl:In at high pressure. In the solid state high pressure cell, F centers are formed in NaCI:In+ which has been irradiated in the near UV C-absorption band of the In+ center. The mechanism is a photothermal release of the excited electron from the In ion through the conduction band to vacancies which exist due to plastic deformation. Subsequently, absorption and emission bands are observed which are characteristic of In++ in NaCl. The In+ and In++ concentrat1ons (as detected by their emissions) were observed to change as functions of irradiation and bleaching time; the kinetics of these processes were investigated quantitatively. The rate coefficients change with pressure; these changes were correlated with observed changes in overlap between various absorption and emission bands in the solid. The thermal energy separation between the excited C-state and the conduction band was found to increase -80% in 70 kbar.U of I OnlyThesi