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 ω≈9meV, corresponding to a scattering vector Q≈2.2nm^-1. At higher Q, the nature of the signal changes rapidly, indicating a crossover to strong scattering. The absence of spectral intensity near ω0ex0ex=0ex0ex0 shows that these excitations remain long-lived compared to what might be inferred from their apparent line widths.