Exciton Luminescence in Mercurous Halide Single Crystals

Abstract

Broad Gaussian luminescence bands with large Stokes shift from free exciton absorption bands have been observed at 11.7 kcm⁻¹ and 11.1 kcm⁻¹ in mercurous halide single crystals of Hg₂I₂ and Hg₂Br₂, respectively at 4.2 K. The excitation spectra and the temperature dependences of the peak energy, the intensity and the decay time have been studied in the above two luminescence bands. A model of self-trapped triplet excitons having two excited states separated by 70 cm⁻¹ is proposed in order to explain the experimental results of the luminescence. The importance of interactions of a mercury diatomic molecule with ten neighboring halogen atoms is suggested from the unpolarized nature of the luminescence.