Rôle of the Jahn-Teller Coupling in the Luminescence Spectra of Fe²⁺ in Zinc-Blende Compounds

The luminescence spectra of Fe2+ in zinc-blende-type II—VI and III—V compounds do not show an equally spaced set of emission lines as predicted by spin orbit interaction in a plain crystalline field. The unequal separation between these lines is a signature of the Jahn-Teller effect in these systems. Attention is focused here on the general trend of the 5 E-derived energy levels providing the end states for the emission transitions. The intervall between the second and third energy levels (γ4 and γ3) is employed as a sensitive test based on the two following characteristics: First, this is the spacing that varies the most; second, the emissions to these levels are usually quite sharp as they involve energies not overlapping with phonon-assisted transitions. This property is studied in the plane [hω,EJT] (energy of the coupling phonon and the Jahn-Teller energy which is directly related to the coupling strength). The general behaviour is then studied under different theoretical conditions, in particular those that maximize the effect. Application of this theory to each real compound is thus possible by choosing the right combination of the two variables. To this end, the examples of luminescent substitutional Fe2+ ions in ZnS, ZnTe, and CdTe are discussed based on published spectra. The main emphasis is placed on new precise measurements of the ZnSe : Fe2+ emission. With crystals containing different iron concentrations, changing line shapes, including self-inversion of several emission lines, have been obtained in the 2600 to 2800 cm-1 spectral range. The properties of the four host/impurity systems are satisfactorily explained while an overall description emerges for the whole family of these compounds from a compilation of the derived coupling parameters

Infrared luminescence and application of a vibronic-coupling Hamiltonian to the level structure of CdTe:Fe²⁺

Samples of crystalline CdTe doped with two 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 the 5T2(D)? 5E(D) infrared transitions of substitutional Fe2+(d6) ions. Especially in the range between 2200 and 2300 cm−1, a rich structure is resolved comprising more lines than predicted from plain crystal-field theory. The explanation of all the important lines is found after introducing a vibronic Jahn-Teller term to the Hamiltonian. A linear coupling between the double-degenerate vibrational mode ε (or γ3) to the electronic orbitals of the atomic multiplet of symmetry 5D leads to the diagonalization of the total Hamiltonian in a set of vibronic functions. Just one free parameter is used in the adjustment: the so-called Jahn-Teller energy representing the strength of the coupling. The corresponding value that we report here is 3 cm−1. The energies thus found are in good agreement with the positions of the observed lines in the spectra. With the final wave functions we can calculate the relative intensities of the most important transitions and approximate theoretical line shape. This is also in good agreement with the experiment. Using these same energies and wave functions a calculation was performed to explain data existing in the literature about far-infrared absorption for the system CdTe:Fe2+. Again, good agreement between experiment and theory is found

Hot lines in the infrared absorption spectra of Fe^2+ in III-V compounds

We attempt an explanation of the main features of the high-temperature infrared absorption spectra of Fe2 in III-V compounds GaAs and GaP . We consider a linear Jahn-Teller interaction with two lattice modes of E symmetry, having energies in the range of acoustical and optical phonons, respectively. The upper vibronic states originating from all electronic states of the free ion must be considered to cope with the many possible transitions that arise at temperatures that populate several low-energy vibronic levels. We use Lanczos- recursion procedures to find energies and wave functions. A comparison with experimental energies and intensities is performed. A discussion comparing present findings with previous results based only on cold lines is also done

Jahn-Teller effect in the emission and absorption spectra of ZnS:Cr^2+ and ZnSe:Cr^2+

The Jahn-Teller effect is invoked to explain the fine structure (isolated zero-phonon lines) observed in both the infrared emission and absorption spectra of substitutional Cr2+ impurities in ZnSe and ZnS. The ground 5D2 term of Cr2+ is split by crystal field into a 5T2 ground multiplet and an excited 5E multiplet. We look at transitions among levels belonging to these two multiplets, which happen to be in the near infrared region. Spin-orbit and spin-spin interactions are taken into account. The Jahn-Teller coupling is introduced as a linear coupling considering both ϵ and τ2 phonons. The Lanczos-recursion procedure with a proper choice of the initial state is used to calculate the vibronic functions and energies. It is found that ϵ modes only lead to intensities that do not agree well with those of the zero-phonon doublet observed both in emission and absorption in the cases of ZnS and ZnSe, while τ2 modes give a good explanation of transition energies and transitions strengths in the same cases. A discussion of the relatively high strength of the vibronic coupling for Cr in comparison with other impurities in the same compounds is also included

Two-modes Jahn-Teller effect in the absorption spectra of Fe^2+ in II-VI and III-V compounds

6Coupling of acoustical and optical modes is introduced to interpret zero-phonon lines in extended absorption spectra of Fe2+ in binary compounds of local symmetry Td. Both cubic II-VI (CdTe, ZnTe, ZnSe, ZnS) and cubic III-V (GaAs, InP, GaP) compounds are included in analysis and calculations. For the case of ZnS:Fe2+, which plays an important role here, interesting experiments are reported. The interpretation of the low-temperature absorption spectra of the seven systems unfolds generalities so all observed lines, as well as the absence of some expected lines, can be identified in the same generic way. In fact, only one parameter is freely varied, which is the coupling constant to one optical mode (additional to the usual acoustical one) which is necessary to explain high-energy lines. The general and consistent explanation of several lines for seven different systems provides a complete picture which allows a deep understanding of vibronic coupling to Fe2+ in binary compounds. The values of coupling constants explaining the experiments are tabulated.reservedmixedO. MUALIN; E.E. VOGEL; M. A. DE ORUE; L. MARTINELLI; G. BEVILACQUA; H.-J. SCHULZO., Mualin; E. E., Vogel; M. A., DE ORUE; L., Martinelli; Bevilacqua, Giuseppe; H. J., Schul