Use of differential impurity induced spectra for the investigation of phonon dispersion relations

The use of differential impurity induced spectra for the investigation of elementary excitations in solids is discussed. It is shown theoretically that the differential spectra are expected to have peaks at energies corresponding to energies of the excitation at critical points of the pure crystal independent of the nature of the impurity used. As an example we present experimental results of the derivative of the intensity of Raman scattered light with respect to wavelength from Tl doped KBr crystals.