Fine Structure of Infrared Absorption Spectra of an Electroluminescent ZnS(CuCl) Crystal during Emission

In an electroluminescent ZnS(Cucl) polytype crystal, we studied the variation of the absorption spectrum in the near infrared, produced by the electric field responsible for electroluminescence. A highly frequency-sensitive detection method is used. When the electric field reaches the electroluminescence threshold, we observe an absorption in the infrared consisting of eight well-defined bands at 1.27, 1.21, 1.13, 1.06, 0.99, 0.93, and 0.87 eV. The temperature, phase, and frequency dependence of ΔII₀ (I₀ is the intensity of the radiation after reflection or transmission of the nonexcited crystal) is analyzed and discussed, taking into acount the lifetime of the Cu⁺⁺ center. Four of the eight peaks are quite broad, and can be attributed to transitions between the valence band and localized levels, and four are sharp and are attributed to internal electronic transitions assisted by phonons.