  X-ray photoelectron diffraction is a powerful tool for determining the
structure of clean and adsorbate-covered surfaces. Extending the technique into
the ultrafast time domain will open the door to studies as diverse as the
direct determination of the electron-phonon coupling strength in solids and the
mapping of atomic motion in surface chemical reactions. Here we demonstrate
time-resolved photoelectron diffraction using ultrashort soft X-ray pulses from
the free electron laser FLASH. We collect Se 3d photoelectron diffraction
patterns over a wide angular range from optically excited Bi$_2$Se$_3$ with a
time resolution of 140 fs. Combining these with multiple scattering simulations
allows us to track the motion of near-surface atoms within the first 3 ps after
triggering a coherent vibration of the A$_{1g}$ optical phonons. Using a
fluence of 4.2 mJ/cm$^2$ from a 1.55 eV pump laser, we find the resulting
coherent vibrational amplitude in the first two interlayer spacings to be on
the order of 1 pm.
