Phonon side bands in the absorption spectra ofNi2+andCo2+in CdI2and PbI2

The absorption and magnetic circular dichroism spectra of the ${}_{}{}^3{}_{}{}^{}A_{2g}^{}{}_{}{}^{}\to {}_{}{}^1{}_{}{}^{}E_g^{}{}_{}{}^{}$ electronic transition of Cd${\mathrm{I}}_2$: ${\mathrm{Ni}}^{2+}$ and Pb${\mathrm{I}}_2$: ${\mathrm{Ni}}^{2+}$ and of the $a{}_{}{}^4{}_{}{}^{}T_{1g}^{}{}_{}{}^{}\to b{}_{}{}^2{}_{}{}^{}T_{1g}^{}{}_{}{}^{}$, ${}_{}{}^2{}_{}{}^{}A_{1g}^{}{}_{}{}^{}$ transition of Cd${\mathrm{I}}_2$: ${\mathrm{Co}}^{2+}$ have been measured. The spectra show a complicated fine structure caused by the coupling with nonlocalized vibrational modes of the host lattice. The theory of this effect is presented. It is shown that the observed vibronic structure images the phonon density of states in the Brillouin zone, modified by matrix elements and selection rules. The contributions of the different phonon branches (acoustic, Raman, and infrared-active modes) are calculated. The spectra show a strong anisotropy, which is due to the presence of static dipoles at the anions; the magnitude of the anisotropy is calculated with use of the polarizable-ion model. The vibronic fine structure is used to deduce the maxima of the phonon density of states of Cd${\mathrm{I}}_2$ and Pb${\mathrm{I}}_2$, and to estimate the phonon dispersion curves.