Ultrafast carrier and phonon dynamics in ion-irradiated graphite

The effect of defects on the dynamics of photoexcited carriers and coherent acoustic phonon in graphite is investigated by means of reflectivity measurements with femtosecond time resolution. Point defects are introduced by irradiating graphite with 5 keV He+ ions. Introduction of the defects enhances the carrier relaxation by opening a decay channel via vacancy states, which competes efficiently with carrier-phonon scattering. The coherent phonon relaxation is also accelerated due to an additional scattering by defects. The linear fluence dependence of the decay rate is understood as scattering of propagating acoustic phonon by single vacancies. (C) 2001 American Institute of Physics.