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 íasOH and the two Evans holes appearing on its shoulder. Spektralna odbojnost monokristalov ima lahko zelo različne in zanimive oblike; mednje sodijo zlasti Evansove luknje. Take oblike smo opisali z modelnim računom z uporabo dielektrične funkcije. Kadar se šibak oscilator pojavi na frekvenčnem območju močnega oscilatorja (to je med njegovima TO in LO frekvencama), se LO-frekvenca šibkega oscilatorja pomakne k višjim frekvencam, preko njegove TO-frekvence. Oscilatorska moč takega nihanja je negativna, kar povzroči padec odbojnosti in s tem Evansovo luknjo v spektru. Ta model smo uspešno uporabili za analizo TE-polariziranega spektra odbojnosti monokristala kalijevega hidrogen sukcinata v ac kristalni ravnini. Monoklinsko kristalno simetrijo smo opisali z enostavnim fenomenološkim modelom in s kompletno simetrijsko analizo. Pri fenomenološkem opisu smo uporabili štiri-parametrični model dielektrične funkcije. Ta omogoča zelo dober opis oblike trakov v vseh spektrih, tudi asimetričnih. Oscilatorske moči nekaterih trakov so kosinusno odvisne od polarizacijskega kota, kar je posledica zelo majhne razlike monoklinskega kristalnega kota od ortorombskega.Zaradi tega fenomenološki opis daje dobre parametre za večino nihanj v spektru. Kompletna simetrijska analiza opiše nihanja v ac ravnini z dielektričnim tenzorjem, v njegovih lastnih smereh pa uporablja tri-parametrični model za dielektrično funkcijo. Tak opis daje zelo dobre rezultate za smeri dipolnih momentov, vendar slabše opiše oblike trakov v spektrih. Obe aproksimaciji opišeta vse osnovne spektralne oblike, široki trak zaradi íasOH in obe Evansovi luknji na njegovih ramenih.