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 <~1ppm and ~200ppm of OH) has been quantitatively investigated in the region 10--100cm^-1 at room temperature. An absorption coefficient {α}({ν}) increased with increasing OH content and a broad peak on a plot of α(ν)/ν² vs {ν}, corresponding to a ``boson peak'' shifted from 41 to 36cm^-1. The OH-related absorption increase {Δ}{α}({ν}), showed a monotonic increase with frequency {ν} in contrast to that previously published. The rate of the absorption increase {Δ}{α}({ν})/{α}({ν}) showed a rapid decrease with frequency obeying a power-law ∝ν^-1.7 between ~17 and 51cm^-1, whereas it decreased very slowly below ~17cm^-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.