Trapping and Annihilation of Electrons and Positive Holes in KCl-TlCl

Abstract

During $\gamma$ irradiation at 77°K, $\mathrm{Cl}_2^{}{}_{}{}^{-}$ and thallium atoms (${\mathrm{Tl}}^0$) are formed in KCl-TlCl crystals. On warming the irradiated crystals, two glow peaks are observed. The low-temperature glow peak at 208°K is caused by the diffusion of $\mathrm{Cl}_2^{}{}_{}{}^{-}$ and the subsequent electron-hole annihilation involving nearby pairs of $\mathrm{Cl}_2^{}{}_{}{}^{-}$ and ${\mathrm{Tl}}^0$. In addition to diffusing into the neighborhood of ${\mathrm{Tl}}^0$, a $\mathrm{Cl}_2^{}{}_{}{}^{-}$ can diffuse into the neighborhood of a ${\mathrm{Tl}}^{+}$ and become retrapped to form ${\mathrm{Tl}}^{++}$. The high-temperature glow peak at 300°K results when electrons are thermally released from thallium atoms into the conduction band and recombination of these electrons and ${\mathrm{Tl}}^{++}$ takes place. Spectral analysis of the low-temperature glow peak shows that it consists of two emission bands with peaks at 410 and 298 nm while the high-temperature glow peak consists only of the 298-nm band. The processes which give rise to these emissions are discussed. The optical absorption of ${\mathrm{Tl}}^0$ and ${\mathrm{Tl}}^{++}$ has been measured. The absorption spectrum of ${\mathrm{Tl}}^0$ consists of very weak bands at 1500 and 1260 nm, a strong band at 380 nm, and weak bands at 250, 300, and 640 nm. ${\mathrm{Tl}}^{++}$ has strong absorption bands at 220, 262, 294, and 364 nm. The concentrations of ${\mathrm{Tl}}^0$ and ${\mathrm{Tl}}^{++}$ produced by irradiation were determined by measuring the decrease in height of the $A$ band of ${\mathrm{Tl}}^{+}$ and thus determining the decrease in the concentration of ${\mathrm{Tl}}^{+}$. From the concentration of ${\mathrm{Tl}}^0$ and ${\mathrm{Tl}}^{++}$ and the optical-absorption spectra of these species it was possible to calculate the oscillator strengths of the ${\mathrm{Tl}}^0$, ${\mathrm{Tl}}^{++}$, and $\mathrm{Cl}_2^{}{}_{}{}^{-}$ bands.