Observation of the crossover to strong scattering of acoustic phonons in densified silica

Inelastic x-ray scattering is used to observe high frequency acoustic excit ations in densified silica glass. Longitudinal acoustic phonons are seen up to an energy <(h)over bar omega> approximate to 9 meV, corresponding to a scattering vector Q approximate to 2.2 nm(-1). At higher Q, the nature of t he signal changes rapidly, indicating a crossover to strong scattering. The absence of spectral intensity near omega = 0 shows that these excitations remain long-lived compared to what might be inferred from their apparent li ne widths.