Excitonic emission from CsI(Na)

Abstract

Photoluminescence in CsI(Na) was studied. This work used nonionizing light sources and obtained the data from well-annealed zone-refined specimens. A series of excitation and emission spectra were measured at temperatures between 300 and 4.2\ifmmode^\circ\else\textdegree\fi{}K. The emission spectra consist of the characteristic blue emission and other low-temperature emissions. The emission spectra were decomposed into the superposition of Gaussian peaks by computer. It is found for the first time that the characteristic emission consists of two bands, centered at 4200 and 3700 \AA{}, respectively. The latter one increased as the temperature decreased. The energies of localized excitons were calculated by employing a Born-Haber cycle. The calculated results were compared with the measured excitation spectra. Each excitation band was identified as the transition of either a free or a localized exciton. The most important one is the exciton created by transferring an electron from an iodine ion to a substitutional sodium ion. This localized exciton called "impurity" exciton is responsible for the excitation band of the characteristic emission. The $\ensuremath{\beta}$ exciton also contributes to this excitation at low temperatures. A configuration-coordinate diagram is proposed to explain the evolution of the two characteristic emission bands. The diagram consists of the ground state and two excited states. Two polarization experiments were carried out in order to understand the symmetry of the characteristic luminescent center. Neither of the two characteristic emission bands was polarizable.