Observation of large multiple scattering effects in ultrafast electron diffraction on monocrystalline silicon

We report on ultrafast electron diffraction on high quality single crystal silicon. The ultrafast dynamics of the Bragg peaks exhibits a giant photoinduced 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 photoexcitation 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 nonequilibrium dynamics of defects in quasiperfect crystals.