Title: Femtosecond mega-electron-volt electron microdiffraction,
Abstract: Instruments to visualize transient structural changes of inhomogeneous
materials on the nanometer scale with atomic spatial and temporal resolution
are demanded to advance materials science, bioscience, and fusion sciences. One
such technique is femtosecond electron microdiffraction, in which a short pulse
of electrons with femtosecond-scale duration is focused into a micron-scale
spot and used to obtain diffraction images to resolve ultrafast structural
dynamics over localized crystalline domain. In this letter, we report the
experimental demonstration of time-resolved mega-electron-volt electron
microdiffraction which achieves a 5 {\mu}m root-mean-square (rms) beam size on
the sample and a 100 fs rms temporal resolution. Using pulses of 10k electrons
at 4.2 MeV energy with a normalized emittance 3 nm-rad, we obtained high
quality diffraction from a single 10 {\mu}m paraffin (C_44 H_90) crystal. The
phonon softening mode in optical-pumped polycrystalline Bi was also
time-resolved, demonstrating the temporal resolution limits of our instrument
design. This new characterization capability will open many research
opportunities in material and biological sciences.