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 less than or equal to 1 ppm and similar to 200 ppm of OH) has been quantitatively investigated in the region 10-100 cm(-1) at room temperature. An absorption coefficient alpha(nu) increased with increasing OH content and a broad peak on a plot of alpha(nu)/nu(2) vs nu, corresponding to a "boson peak" shifted from 41 to 36 cm(-1). The OH-related absorption increase Delta alpha(nu), showed a monotonic increase with frequency nu in contrast to that previously published. The rate of the absorption increase Delta alpha(nu)/alpha(nu) showed a rapid decrease with frequency obeying a power-law proportional to alpha(-1.7) between similar to 17 and 51 cm(-1), whereas it decreased very slowly below similar to 17 cm(-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.