s- and p-polarized infrared specular reflectance of vitreous silica at oblique incidences: Detection of LO modes

Infrared (IR) specular reflectance spectra of a semi-infinite sample of vit reous silica (v-SiO2) were obtained with title use of both s- and p-polariz ed Light and oblique incidence angles. The optical constants of the materia l and hence its longitudinal optic/transverse optic (LO-TO) functions were determined through the Kramers-Kronig analysis (KKA) of its s-polarized 20 degrees off-normal reflectance spectrum, p-Polarized spectra had their refl ection maxima blue-shifting as the incidence angle increased, while they re mained unchanged for the s-polarized spectra. Since an LO mode generally Li es at wavenumbers higher than its respective TO mode, such a blue shift may be due to the detection of the LO mode in addition to the TO mode as incid ence angle increased. The only exception to this observation was the high-f requency shoulder, which underwent a sharp intensification as the incidence increased. The present work shows that it is indeed brought about by the w eakly IR active asymmetrical mode (AS2) hut only because it takes place imm ediately after the intense AS1 mode, which causes the refraction index spec trum to have a broad dip below unity. Such a dip is proven to be responsibl e for the sharp increase in the high-frequency shoulder of the reflectance spectra.