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Title:Optical absorption of pure and mixed silver halides a low temperatures
Author(s):Joesten, Burdette Lee
Doctoral Committee Chair(s):Brown, Frederick C.
Department / Program:Physics
Subject(s):optical absorption
silver halides
AgCl AgBr mixed crystals
Abstract:The low level indirect optical absorption of pure AgCl and AgBr has been measured between 4.20 K and0 77 K . The corresponding indirect optical absorption of AgCl~AgBr mixed crystals has also been measured in the same temperature range. The temperature dependence of the absorption spectra of pure AgCl and AgBr is explained by the theory of phonon~assisted indirect transitions consistent with an electronic band structure with a conduction band of standard form at k = 0 and a valence band with at least one - maximum away from k == O. For pure Agel,'! the temperature dependence indicates that phonons of two energies are important in the indirect transitions. The equivalent phonon temperatures are 95°K ~ 20K o 0 for the primary phonon and 154 K ~ 9 K for the second phonon. The 4.7°K absorption spectrum of AgCl has structure which suggests that the spin=orbit splitting at the maximum of the valence band in the (111) direction is 0.025 eVa The temperature dependence of the AgBr absorption spectrum is explained primarily by indirect transitions with the emission or 40 0 absorption of 9 K ± 2 K phonons. There is some indication that phonons near 139° K ~ 4° K also ' participate in the indirect transitions. At 4.20K, addition of AgBr to AgCl causes the absorption edge to shift continuously from the pure AgCl threshold at 3.252 eV to the pure AgBr threshold at 2.691 eV. ~he effect of the alloying on the pure crystal absorption spectra is somewhat different for the two extremes of mixing. In AgBr-rich mixed crystals ( > 90 mol ~ AgBr in Agel) a temperature independent component is observed with a sharp threshold at an energy precisely half-way between the indirect phonon emission and absorption thresholds (which still appear with phonon energy 0.0082 eV as in pure AgBr). This suggests that in the alloy non-vertical transitions occur without the aid of phonons to conserve crystal momentum. For dilute concentrations of AgBr in AgCl « 1 mol~) a weak shoulder and tail extends approximately 0.06 eV to longer wavelengths from the phonon-assisted indirect threshold. Above 2 mol % AgBr in AgCl the absorption coefficient increases steeply to values considerably greater than that of pure AgBr at the same wavelength.
Issue Date:1966
Genre:Dissertation / Thesis
Rights Information:1966 Burdette Lee Joesten
Date Available in IDEALS:2011-05-27
Identifier in Online Catalog:6119815

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