Dispersion analysis of polarized bulk reflectance spectra of potassium hydrogen succinate monocrystal.

Reflectance spectra may show very different and interesting shapes. The most interesting may be connected to Evans holes. Model calculations to reproduce this effect were done applying model dielectric function functions. When a weak oscillator appears within the frequency region of a strong one (i.e. between its TO and LO frequencies), its LO-mode frequency shifts towards higher wavenumbers. As a consequence, the oscillator strength of the corresponding weak mode becomes negative which produces a dip (Evans hole) in the reflectance spectrum. This model was successfully applied to analyze polarized reflectance spectra of potassium hydrogen succinate monocrystal in the ac crystal plane. The crystal symmetry was described by phenomenological model and by full symmetry consideration. The phenomenological approach applies the four-parametric model for dielectric function which allows good evaluation of asymmetric reflectance bands. A cosine dependence of oscillator strength on polarization angle was obtained for some bands, most likely due to small deviation of the crystal symmetry from orthorhombic. This is why the phenomenological approach provides good parameters for almost all phonon modes. The full symmetry consideration applies real description of dielectric tensor in the ac plane and the three-parameter model for dielectric function. This model could give very accurate directions of transition dipole moments, but cannot fit asymmetric bands well to measured reflectance. Both approaches describe all basic spectral features, the wide reflectance of asOH and the two Evans holes appearing on its shoulder.