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

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 $ħ\omega \approx 9\mathrm{meV}$, corresponding to a scattering vector $Q\approx 2.2{\mathrm{nm}}^{-1\mathrm{}}$. At higher $Q$, the nature of the signal changes rapidly, indicating a crossover to strong scattering. The absence of spectral intensity near $\omega =0\mathrm{}$ shows that these excitations remain long-lived compared to what might be inferred from their apparent line widths.