Phonon side bands in the absorption spectra of nickel(2+) and cobalt(2+) in cadmium iodide and lead(II) iodide.

The absorption and magnetic CD spectra of the 3A2g → 1Eg electronic transition of CdI2:Ni2+ and PbI2:Ni2+ and of the a4T1g → b2T1g, 2A1g transition of CdI2:Co2+ were 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. The obsd. vibronic structure images the phonon d. of states in the Brillouin zone, modified by matrix elements and selection rules. The contributions of the different phonon branches (acoustic, Raman, and IR-active modes) are calcd. The spectra show a strong anisotropy, which is due to the presence of static dipoles at the anions; the magnitude of the anisotropy is calcd. with use of the polarizable-ion model. The vibronic fine structure is used to deduce the max. of the phonon d. of states of CdI2 and PbI2, and to est. the phonon dispersion curves. [on SciFinder(R)]