Reply to "Comment on `Infrared-reflectance spectra of heat-treated, sol-gel-derived silica"'

Previously reported infrared spectra of amorphous Si${\mathrm{O}}_2$ have been discussed in order to clarify the origin of the shoulder at ∼ 1200 ${\mathrm{cm}}^{-1\mathrm{}}$ in the transmission spectra and of the high-frequency band at ∼ 1260 ${\mathrm{cm}}^{-1\mathrm{}}$ in the reflection spectra. Infrared measurements at oblique incidence with $s$- and $p$-polarized light show that the high-frequency response of silica (between ∼ 1000 and 1300 ${\mathrm{cm}}^{-1\mathrm{}}$) originates from two coupled modes A${\mathrm{S}}_1$ and A${\mathrm{S}}_2$. The polarization characteristics indicate that the features at ∼ 1200 ${\mathrm{cm}}^{-1\mathrm{}}$ and ∼ 1260 ${\mathrm{cm}}^{-1\mathrm{}}$ have different origin and that it is appropriate to attribute to them TO (A${\mathrm{S}}_2$) and LO (A${\mathrm{S}}_1$) 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)].