Ultrafast electron diffraction at surfaces after laser excitation

Ultrafast electron diffraction in a RHEED setup is used to determine the dynamics of surface temperature of an epitaxial thin Bismuth-film on a Si(0 0 1) substrate upon fs-laser excitation. A transient temperature rise by 120 K is followed by a slow exponential cooling with time constant 640 ps. The surface cooling rate deviates from simple heat diffusion and is dominated by total internal reflection of ballistic phonons at the Bi/Si-interface which determines the thermal properties of the hetero-system.