Phonon-phonon interactions in photoexcited graphite studied by ultrafast electron diffraction

We investigated phonon-phonon interactions in photoexcited single-crystalline graphite by ultrafast electron diffraction. Transient electron diffraction profiles from a 35 nm graphite film were observed following laser excitation. Changes in intensities of diffraction spots revealed a two-exponential relaxation process with 0.5 ps and {~}8 ps time scales. Furthermore, the transient thermal diffuse scattering signal showed evidence for population of phonons across the {Γ}K and {Γ}M branches of the phonon dispersion spectrum on a 2 ps time scale. Based on these dynamics, we conclude that the fast relaxation process corresponds to decay of strongly coupled optical phonons, and the slow relaxation process to redistribution of phonon energy to the equilibrium thermal distribution.