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Abstract

In Hg₂Cl₂ crystals broad luminescence bands excited with UV light have been studied in the spectral range 0.8–2.25 eV. The measurements have been performed on as-grown crystals and on crystals previously exposed to UV light at RT. The emission spectra consist of six bands peaking at 0.82, 1.06, 1.23, 1.39, 1.51, and 1.72 eV with FWHM of 0.21–0.33 eV. Their intensities depend conspicuously on the concentration of the photochemical products induced by irradiation of the crystals at RT. It is concluded that the infra-red luminescence of Hg₂Cl₂ originates from crystal defects, especially that complexes (HgCl_xBr_3−x)− and (HgCl_xBr_4−x)²⁻ are responsible for these emissions. Excitation of the luminescence can occur: (i) via excitons in the Hg₂Cl₂ matrix, (ii) via excited states of Hg₂Br₂ molecules, and (iii) resonantly through the excitation bands of defect centres. A configurational coordinate diagram for the emission observed at 1.51 eV is proposed. A model for the temperature quenching of luminescence and energy transfer between two different kinds of centres is used to explain the more complicated temperature dependence of this luminescence.