Nonequilibrium optical phonons are generated in graphite following the
excitation of electron-hole pairs with a femtosecond laser pulse. Their energy
relaxation is probed by means of terahertz pulses. We find that the hot-phonon
lifetime increases by a factor of 2 when the sample temperature decreases from
300 to 5 K. These results suggest that the energy relaxation in graphite at
room temperature and above is dominated by the anharmonic decay of hot
A′1phonons at the K point into acoustic phonons with energies of about 10 meV
