RAMAN AND BRILLOUIN SPECTRA OF VITREOUS SILICA

Abstract

Author Institution: Divisions of Pure Physics and Chemistry, National Research CouncilThe heat capacity of vitreous silica in the region $T <20^{\circ} K$ is very much larger than that observed for simple crystals. In order to interpret this unusual behaviour some spectroscopic studies have been made. The Brillouin spectrum excited by $\lambda 2536.5$ of $Hg^{198}$ was photographed in the third order of a 35-ft. grating. Lines due to scattering by longitudinal waves were observed, together with much weaker lines attributed to transverse waves. Their frequency shifts form the exciting line are 1.68 and $1.04 cm^{-1}$ respectively. The shifts give directly the frequencies of the Debye waves producing the scattering namely, $5.03 \times 10^{10} Sec^{-1}$ and $3.12 \times 10^{10} Sec^{-1}$. Their velocities are in excellent agreement with the values determined by acoustic methods at a frequency of $10^{7} sec^{-1}$. These results show that dispersion of lattice waves in vitreous silica is not significant up to frequencies of about $5 \times 10^{10} sec^{-1}$. The Raman spectrum excited by Hg 2537 was photographed at low dispersion and in the fourth order of a 21-ft. grating. Its most prominent feature is an intense continuum starting below $8 cm^{-1}$ and extending to about $560 cm^{-1}$ where it has a sharp cut-off. These results give direct evidence for low frequency optical modes whose presence can account for the observed heat capacity. The origin of the spectrum is still a matter for speculation