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

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Abstract

During $\ensuremath{\gamma}$ irradiation at 77\ifmmode^\circ\else\textdegree\fi{}K, $\mathrm{Cl}_{2}^{}{}_{}{}^{\ensuremath{-}}$ 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\ifmmode^\circ\else\textdegree\fi{}K is caused by the diffusion of $\mathrm{Cl}_{2}^{}{}_{}{}^{\ensuremath{-}}$ and the subsequent electron-hole annihilation involving nearby pairs of $\mathrm{Cl}_{2}^{}{}_{}{}^{\ensuremath{-}}$ and ${\mathrm{Tl}}^{0}$. In addition to diffusing into the neighborhood of ${\mathrm{Tl}}^{0}$, a $\mathrm{Cl}_{2}^{}{}_{}{}^{\ensuremath{-}}$ can diffuse into the neighborhood of a ${\mathrm{Tl}}^{+}$ and become retrapped to form ${\mathrm{Tl}}^{++}$. The high-temperature glow peak at 300\ifmmode^\circ\else\textdegree\fi{}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 $\mathrm{Cl}_{2}^{}{}_{}{}^{\ensuremath{-}}$ bands.

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