Effect of OH content on the far-infrared absorption and low-energy states in silica glass

The far-infrared absorption for two types of silica glasses (containing $<~1\mathrm{ppm}$ and $\sim 200\mathrm{ppm}$ of OH) has been quantitatively investigated in the region $10–100{\mathrm{cm}}^{\mathrm{-}1}$ at room temperature. An absorption coefficient α(ν) increased with increasing OH content and a broad peak on a plot of $\alpha \left(\nu \right)/{\nu }^2$ vs ν, corresponding to a “boson peak” shifted from 41 to $36{\mathrm{cm}}^{\mathrm{-}1}.$ The OH-related absorption increase Δα(ν), showed a monotonic increase with frequency ν in contrast to that previously published. The rate of the absorption increase Δα(ν)/α(ν) showed a rapid decrease with frequency obeying a power-law $\propto {\nu }^{-1.7}$ between $\sim 17$ and $51{\mathrm{cm}}^{\mathrm{-}1},$ whereas it decreased very slowly below $\sim 17{\mathrm{cm}}^{\mathrm{-}1}.$ It is suggested on the basis of a noncontinuous network model for the glass that OH ions are not uniformly distributed in silica glass. The light-vibration coupling coefficient determined experimentally is briefly discussed by some models proposed before.