Detection of LO modes in glass by infrared reflection spectroscopy at oblique incidence

The occurrence of longitudinal-transverse splittings in the optic-vibrational-mode frequencies of partially ionic glasses has previously been observed in several glass systems and was also theoretically confirmed. The transverse-optic (TO) and longitudinal-optic (LO) components are usually derived from near-normal infrared-specular-reflectivity spectra by Kramers-Kronig analysis, and the TO frequencies often coincide with minima in the transmission spectrum. The approximate positions of the high-frequency LO modes of vitreous silica (v-${\mathrm{SiO}}_2$) and two more ionic modified fluoride glasses based on ${\mathrm{ZrF}}_4$ (ZBLA glass) and ${\mathrm{ZnF}}_2$-${\mathrm{CdF}}_2$ (ZnCd glass) have been directly obtained from infrared-external-specular-reflectivity spectra at oblique incidence. Fourier-transform infrared reflectivity spectra were recorded at incidence angles between 10° and 70° off normal, and the results are compared with Kramers-Kronig analysis of the near-normal (10°) incidence data and polarized Raman spectra of the same glasses. The reflection spectra at ≳50° off normal contained peaks corresponding very approximately to the LO vibrational modes, while the peaks in the Raman spectra did not generally coincide with either TO or LO frequencies of the glasses. The high-frequency LO modes were experimentally observed at 1286, ∼607, and 617 ${\mathrm{cm}}^{\mathrm{-}1}$, for v-${\mathrm{SiO}}_2$, ZBLA, and ZnCd glasses, respectively, compared to Kramers-Kronig values calculated at 1266, 625, and ∼623 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively. Ionicity was found to be an important factor in determining the extent of LO-TO splittings. The original Berreman argument for the case of transmission through thin crystal films may be extended to reflection from bulk glasses.