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The EPR spectra of ${\mathrm{Cr}}^{2+}$ in cubic ZnS, ZnSe, ZnTe, and CdTe, and in hexagonal ZnS and CdS are reported. For each, $\ensuremath{\Delta}M=\ifmmode\pm\else\textpm\fi{}4, \ifmmode\pm\else\textpm\fi{}2, \ifmmode\pm\else\textpm\fi{}1$ transitions are observed allowing complete analysis of the $S=2$ spin Hamiltonian. A static tetragonal Jahn-Teller distortion is observed in all cases. Stress-alignment studies are described which allow a direct estimate of the Jahn-Teller coupling coefficients. From these, the magnitudes of the Jahn-Teller energies are estimated to be \ensuremath{\sim}500 ${\mathrm{cm}}^{\ensuremath{-}1}$. A ligand-field model is developed which satisfactorily accounts for the fine-structure parameters and their large variation from one crystal to another. In this treatment, ligand contributions to the spin-orbit interaction with excited ${d}^{4}$ terms are also included and found to be important. Strain-coupling coefficients, describing the changes in the fine-structure terms under externally applied stress, have been experimentally determined for CdTe (${\mathrm{Cr}}^{2+}$). A simple theory for this effect is developed which includes ligand contributions to the spin-orbit interactions but relies on point-ion estimates for the strain matrix elements. Agreement with experiment is reasonable for the tetragonal coefficients, but the wrong sign is predicted for the trigonal coefficients. The calculated trigonal coefficients for CdS do, however, satisfactorily account for the small departures from tetragonal symmetry observed in the spin Hamiltonian for ${\mathrm{Cr}}^{2+}$ in this hexagonal wurtzite lattice.

A calculation of the $^{5}E$ energy levels of ${\mathrm{Fe}}^{2+}$ in CdTe has been performed taking into account coupling between the electron system of the impurity ion and the vibrations of the host lattice. The coupling is approximated to a single pair of $E$ modes corresponding to the $\mathrm{TA}(L)$ phonons. We find that even a small electron-phonon coupling not only displaces the energy levels from those of crystal-field theory, but also introduces additional transitions. We get qualitative agreement with the experimental observations for an effective spin-orbit splitting of 20.8 ${\mathrm{cm}}^{\ensuremath{-}1}$ and a Jahn-Teller energy of 4.2 ${\mathrm{cm}}^{\ensuremath{-}1}$. We also find that we get better agreement between theory and experiment if we take spin-spin interactions into account.

We have investigated several optical transitions between the $^{5}E$ energy levels of the orbital ground state of substitutional ${\mathrm{Fe}}^{2+}$ in natural crystals of cubic ZnS for magnetic inductions up to 14 T (140 kG) at liquid-He temperatures. In these experiments we have used both gas lasers and a Fourier-transform Michelson spectrometer covering the range 10-100 ${\mathrm{cm}}^{\ensuremath{-}1}$ wave numbers. The results show that simple crystal-field theory is adequate to account for the observed energy levels and their oscillator strengths as a function of the applied magnetic field. There is also evidence for local modes at 43 and 63 ${\mathrm{cm}}^{\ensuremath{-}1}$ wave numbers caused by trace impurities other than Fe.

The optical absorption spectra of substitutional ${\mathrm{Cr}}^{2+}$ ($3{d}^{4}$) in single crystals of ZnS (cubic and mixed cubic-and-hexagonal), ZnSe, ZnTe, CdS, and CdTe have been measured for Cr concentrations of ${10}^{18}$-${10}^{20}$ ${\mathrm{cm}}^{\ensuremath{-}3}$ and at 2, 15, and 300\ifmmode^\circ\else\textdegree\fi{}K in the wave-number range $5\ensuremath{\le}\overline{\ensuremath{\nu}}\ensuremath{\le}30000$ ${\mathrm{cm}}^{\ensuremath{-}1}$. The room-temperature spectrum in the range $500\ensuremath{\le}\overline{\ensuremath{\nu}}\ensuremath{\le}30000$ ${\mathrm{cm}}^{\ensuremath{-}1}$ is dominated by a broad absorption band at 5500 ${\mathrm{cm}}^{\ensuremath{-}1}$ arising from the $^{5}T_{2}\ensuremath{\rightarrow}^{5}E$ transition. These two levels originate from the crystal-field splitting of the $^{5}D$ free-ion ground state. At helium temperatures distinct lines appear on the low-energy side of the $^{5}T_{2}\ensuremath{\rightarrow}^{5}E_{2}$ absorption band in all samples except for CdTe. These lines are associated with zero-phonon transitions and phonon-assisted transitions. As the temperature is increased, the relative intensities of some of the lines change due to the thermal population of low-lying levels belonging to the orbital ${T}_{2}$ state. In the case of ZnSe these levels have been studied in the far infrared in the range $5\ensuremath{\le}\overline{\ensuremath{\nu}}\ensuremath{\le}200$ ${\mathrm{cm}}^{\ensuremath{-}1}$, both with and without a magnetic field. An interpretation of the results is made based on crystal-field theory with a strong, static, Jahn-Teller distortion of $E$ symmetry in the $^{5}T_{2}$ orbital ground state. There is little or no Jahn-Teller effect in the $^{5}E$ orbital excited state. The optical transitions exhibit a strong coupling to $E$-mode phonons of average energy \ensuremath{\sim}70 ${\mathrm{cm}}^{\ensuremath{-}1}$.

The optical absorption in Cr doped ZnSe has been measured for photon wavenumbes,, ν , from 2 cm−1 to the bandedge at about 20 000 cm−1 for temperatures from 1.2 to 300°K. The Cr produces an extremely sharp absorption line at ν = 7.43 cm −1 , additional broad absorption lines between 50 and 100 cm−1, a broad absorption band at 5500 cm−1, and some further structure at higher energies. As we will show, these results support the conclusion of earlier paramagnetic resonance studies that a static Jahn-Teller distortion of E symmetry is present in the ground state for Cr2+ ions in ZnSe.

An isotropic paramagnetic resonance of Fe2+in KMgF3 has been observed at low temperature. From the relatively sharp resonances associated with the double quantum transition and with the half field line it is deduced that the g-value is 3.36 instead of the value of 3.53 predicted by second order crystal field theory. The difference is attributed to a combination of the dynamic Jahn-Teller effect and the effect of covalency both of which reduce the orbital contribution to the g-value.