Infrared luminescence and vibronic coupling in ZnTe:Fe²⁺

Samples of crystalline ZnTe with different concentrations of iron were prepared by the vertical high-pressure Bridgman method. Absorption and emission spectra were recorded at liquid-helium temperature in the region of the⁵T₂(D)⇌⁵E(D) infrared transitions of substitutional Fe²⁺(d⁶) ions. In the range between 2400 and 2520 cm^-1 a rich structure is observed showing more lines than predicted by plain crystal-field theory. The explanation of all these lines is found after introducing a vibronic Jahn-Teller term to the Hamiltonian. A linear coupling between the doubly degenerate vibrational mode e{open} to the electronic orbitals of the ionic multiplet⁵D is added to the electronic and vibrational terms of the Hamiltonian. A diagonalization follows using just one free parameter: the Jahn-Teller energy representing the strength of the coupling. The corresponding value is 3 cm^-1, which is identical to the one already reported for CdTe: Fe²⁺. The calculated spectrum is in good agreement with the one determined experimentally. Measurements of the absorption spectra support conclusions drawn earlier about a Jahn-Teller coupling also for the excited multiplet. Finally a prediction of how the far-infrared would look like is also given.