  We report a benchmark experiment that demonstrates shortening of hard x-ray
pulses in a synchrotron-based optical pump - x-ray probe experiment. The pulse
shortening device, a picosecond Bragg switch, reduces the temporal resolution
of an incident x-ray pulse to 7.5 ps. We employ the Bragg switch to monitor
propagating sound waves in nanometer-thin epitaxial films. With the
experimental data we infer pulse duration, diffraction efficiency and switching
contrast of the device. A detailed efficiency analysis shows, that the switch
can deliver up to 1010 photons/sec in high-repetition rate synchrotron
experiments.
