  We report on ultrafast electron diffraction on high quality single crystal
silicon. The ultrafast dynamics of the Bragg peaks exhibits a giant
photo-induced response which can only be explained in the framework of
dynamical diffraction theory, taking into account multiple scattering of the
probing electrons in the sample. In particular, we show that lattice heating
following photo-excitation can cause an unexpected increase of the Bragg peak
intensities, in contradiction with the well-known Debye-Waller effect. We
anticipate that multiple scattering should be systematically considered in
ultrafast electron diffraction on high quality crystals as it dominates the
Bragg peak dynamics. In addition, taking into account multiple scattering
effects opens the way to quantitative studies of non-equilibrium dynamics of
defects in quasi-perfect crystals.
