Quantitative analysis of the effect of disorder-induced mode coupling on infrared absorption in silica

The infrared-absorption spectrum of a-${\mathrm{SiO}}_2$ is analyzed in terms of its transverse-optic (TO) and longitudinal-optic (LO) vibrational modes. It is shown that the independent-oscillator model for the a-${\mathrm{SiO}}_2$ dielectric function fails to yield a consistent value of mode strength for the optically active oxygen asymmetric stretch (${\mathrm{AS}}_1$) TO mode at 1076 ${\mathrm{cm}}^{\mathrm{-}1}$ (in-phase motion of adjacent oxygen atoms) when different but equivalent methods of measurement and analysis are used. This inconsistency is resolved by introducing disorder-induced mechanical coupling between the ${\mathrm{AS}}_1$ mode and the relatively optically inactive oxygen asymmetric stretch (${\mathrm{AS}}_2$) mode (out-of-phase motion of adjacent oxygen atoms) into the oscillator model. Coupled ${\mathrm{AS}}_1$- and ${\mathrm{AS}}_2$-mode LO-TO frequency pairs are experimentally observed as peaks at approximately 1256–1076 ${\mathrm{cm}}^{\mathrm{-}1}$ and 1160–1200 ${\mathrm{cm}}^{\mathrm{-}1}$, respectively, in oblique-incidence p-polarized absorption spectra of thin a-${\mathrm{SiO}}_2$ films grown thermally on c-Si wafers.