Infrared-reflectance spectra of heat-treated, sol-gel-derived silica - Reply

Previously reported infrared spectra of amorphous SiO2 have been discussed in order to clarify the origin of the shoulder at similar to 1200 cm(-1) in the transmission spectra and of the high-frequency band at similar to 1260 cm(-1) in the reflection spectra. Infrared measurements at oblique incidence with s- and p-polarized light show that the high-frequency response of silica (between similar to 1000 and 1300 cm(-1)) originates from two coupled modes AS(1) and AS(2). The polarization characteristics indicate that the features at similar to 1200 cm(-1) and similar to 1260 cm(-1) have different origin and that it is appropriate to attribute to them TO (AS(2)) and LO (AS(1)) character, respectively. This assignment was found to be consistent with the infrared-reflectance measurements at oblique incidence reported by Almeida [Phys. Rev. B 45, 161 (1992)] and with those at nearly normal incidence reported by Kamitsos et al. and analyzed by Kramers-Kronig transformation [Phys. Rev. B 48, 12 499 (1993)]. The two-mode interpretation of the high-frequency response of silica was found to be consistent with the trends of the effective transverse-optic and longitudinal-optic frequencies, which were calculated using the second-moment expressions of Noh and Sievers [Phys. Rev. Lett. 17, 1800 (1989)].