The attenuation of longitudinal acoustic phonons up to frequencies nearing $250\mathrm{GHz}$ is measured in vitreous silica with a picosecond optical technique. By taking advantage of interferences on the probe beam, difficulties encountered in early pioneering experiments are alleviated. Sound damping at $250\mathrm{GHz}$ and room temperature is consistent with relaxation dominated by anharmonic interactions with the thermal bath, extending optical Brillouin scattering data. Our result is at variance with claims of a recent deep-UV experiment which reported a rapid damping increase beyond $100\mathrm{GHz}$. A comprehensive picture of the frequency dependence of sound attenuation in $v\text{−}\mathrm{Si}{\mathrm{O}}_2$ can be proposed.

• Received 25 January 2008

DOI:https://doi.org/10.1103/PhysRevB.77.100201