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Low temperature optical spectra are studied on AgBr:Cl - in comparison with AgBr. Contrary to AgBr:I - , no bound excitons at chlorine are formed in AgBr:Cl - . Absorption spectra of AgBr:Cl - are similar to AgBr, except for existence of exciton absorption without phonon assistance. Photoluminescence spectra of AgBr and AgBr:Cl - contain emission band due to decay of free exciton. This observation leads one to conclude that positive holes in AgBr are not self-trapped even at 2°K. Comparison of free exciton transition energies between absorption and emission spectra reveals existence of two kinds of phonons assisting the indirect transition. Various extrinsic emission bands are also observed. One of them corresponds to shallow-bound exciton at Cd 2+ -type impurity and the other to deep-bound exciton at residual iodine. Transition energies of free and bound excitons, except iodine-bound exciton, shift to higher energy in proportion to chlorine concentration. These results can be explained by virtual crystal model for AgBr:Cl - in which valence band is lowered in comparison with AgBr.
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Kanzaki et al. (1970) studied this question.
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