The absorption spectra of KIz, CdI,:Ni2+, and CoI, in the wave-number region 4000 to 16000 cm-l are reported. The spectra are interpreted in terms of d-d transitions of NiZ+ and Co2+ ions. Parameters of the electron repulsion, cubic crystal field, and spin-orbit coupling are estimated. From the latter the transferred spin densities are deduced and compared with Mossbauer data. Birefringence experiments as a function of the temperature show the presence of two phase transitions in XI,.Nous avons mesure les spectres d'absorption de NiI,, CdI,:Ni2+ et CoI, dans la region de 4000 B 16000 em-l. Les spectres ont Ate interpret6 comme transitions d-d des ions Ni2+ et Co2+. A partir des spectres on a determine les parametres de la repulsion interelectronique, du champ cristailin cubique et de la couplage spin-orbite. La densite de spin sup l'ion de iode est calcule et comparu avec le spectre Mossbauer. Experiments de la refraction double en fonction de la temperature montrent la presence de deux transitions de phase dans NiI,.
The absorption and magnetic circular dichroism spectra of the A2g 'Eg electronic transition of CdI2'. Ni + and PbI2.Ni + and of the a T~g 6 T&g A]g transition of CdI2'. Co + 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 CdI2 and PbI2, and to estimate the phonon dispersion curves.
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