  We measure the lifetime of the zone-center 340 GHz longitudinal phonon mode
in a GaAs-AlAs superlattice excited and probed with femtosecond laser pulses.
By comparing measurements conducted at room temperature and liquid nitrogen
temperature we separate the intrinsic (phonon-phonon scattering) and extrinsic
contributions to phonon relaxation. The estimated room temperature intrinsic
lifetime of 0.95 ns is compared to available calculations and experimental data
for bulk GaAs. We conclude that ~0.3 THz phonons are in the transition zone
between Akhiezer and Landau-Rumer regimes of phonon-phonon relaxation at room
temperature.
