Edge Emissions and Broad Band Emissions in Thallous Halides

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The emission spectra are measured in pure TlBr and TlCl crystals as well as in the mixed crystals of TlBr(Cl), TlBr(I), TlCl(Br) and TlCl(I). Sharp emission lines near the absorption edge are observed in TlBr and TlCl. High energy lines among them are examined with the aid of the exciton state calculation and are interpreted as the radiative decay of the indirect forbidden X₆⁺×R₆⁻ excitons. In the halogen-doped crystals, the emissions due to the zero phonon decay of the X₆⁺×R₆⁻ excitons are observed. The broad band emissions observed in the long wavelength region in the halogen-doped crystals are assigned to the decays of the relaxed excitons localized at the added halogen ions.