  We report the experimental demonstration of femtosecond electron diffraction
using high-brightness MeV electron beams. High-quality, single-shot electron
diffraction patterns for both polycrystalline aluminum and single-crystal
1T-TaS2 are obtained utilizing a 5 femto-Coulomb (~3x10^4 electrons) pulse of
electrons at 2.8 MeV. The high quality of the electron diffraction patterns
confirm that electron beam has a normalized emittance of ~50 nm-rad. The
corresponding transverse and longitudinal coherence length are ~11 nm and ~2.5
nm, respectively. The timing jitter between the pump laser and probe electron
beam was found to be ~ 100 fs (rms). The temporal resolution is demonstrated by
observing the evolution of Bragg and superlattice peaks of 1T-TaS2 following an
800 nm optical pump and was found to be 130 fs. Our results demonstrate the
advantages of MeV electron diffraction: such as longer coherent lengths, large
scattering cross-section and larger signal-to-noise ratio, and the feasibility
of ultimately realizing 10 fs time-resolved electron diffraction.
