Observation of the Crossover to Strong Scattering of Acoustic Phonons in Densified Silica

Abstract

Inelastic x-ray scattering is used to observe high frequency acoustic excitations in densified silica glass. Longitudinal acoustic phonons are seen up to an energy $\ensuremath{\Elzxh}\ensuremath{\omega}\ensuremath{\approx}9\mathrm{meV}$, corresponding to a scattering vector $Q\ensuremath{\approx}2.2{\mathrm{nm}}^{\ensuremath{-}1}$. At higher $Q$, the nature of the signal changes rapidly, indicating a crossover to strong scattering. The absence of spectral intensity near $\ensuremath{\omega}\phantom{\rule{0ex}{0ex}}=\phantom{\rule{0ex}{0ex}}0$ shows that these excitations remain long-lived compared to what might be inferred from their apparent line widths.