Temperature Dependence of Absorption Bands of (NH₄)₂SO₄:Tl Crystal

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The absorption spectra of ammonium sulphate, (NH₄)₂SO₄, crystals containing a small amount of thallous ions have been measured in the energy region from 20000 to 60000 cm⁻¹. At 15°C only one broad band is found for all of a, b and c-polarized lights below the fundamental absorption edge of the host crystals. However, at −196°C the band for c-polarized light is split into two, 46450 and 47750 cm⁻¹, and for a-polarized light there appears a shoulder in the higher energy side of the band. These absorption bands are attributed to the superposition of the transitions between the molecular orbitals in the two unequivalent Tl-(SO₄)_n complexes. The temperature dependence of the absorption spectra is explained by that of the molecular orbital energies of the complexes and that of the crystal fields of (NH₄)₂SO₄ crystals.