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 $\mathrm{Bi}{}_2\mathrm{Se}{}_3$ 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 $4.2\mathrm{mJ}/{\mathrm{cm}}^2$ 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 0.01 Å.

• Received 25 May 2022
• Revised 20 October 2022
• Accepted 16 November 2022

DOI:https://doi.org/10.1103/PhysRevB.106.L201409