  The dephasing of coherent longitudinal optical (LO) phonons in ion-irradiated
GaP has been investigated with a femtosecond pump-probe technique based on
electro-optic sampling. The dephasing time of the coherent LO phonon is found
to be dramatically prolonged by the introduction of a small amount of defects
by means of Ga-ion irradiation. The maximum dephasing time observed at room
temperature is 9.1 ps at a Ga$^{+}$ ion dose of 10$^{13}$/cm$^{2}$, which is
significantly longer than the value of 8.3 ps for GaP before ion irradiation.
The longer dephasing time is explained in terms of the suppression of
electron-LO-phonon scattering by the presence of defect-induced deep levels.
