  Visualizing ultrafast dynamics at the atomic scale requires time-resolved
pump-probe characterization with femtosecond temporal resolution. For
single-shot ultrafast electron diffraction (UED) with fully relativistic
electron bunch probes, existing techniques are limited by the achievable
electron probe bunch length, charge, and timing jitter. We present the first
experimental demonstration of pump-probe UED with THz-driven compression and
time-stamping that enable UED probes with unprecedented temporal resolution.
This technique utilizes two counter-propagating quasi-single-cycle THz pulses
generated from two OH-1 organic crystals coupled into an optimized THz
compressor structure. Ultrafast dynamics of photoexcited bismuth films show an
improved temporal resolution from 178 fs down to 85 fs when the THz-compressed
UED probes are used with no time-stamping correction. Furthermore, we use a
novel time-stamping technique to reveal transient oscillations in the dynamical
response of THz-excited single-crystal gold films previously inaccessible by
standard UED, achieving a time-stamped temporal resolution down to 5 fs.
