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 Bi2Se3 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 A1g optical phonons. Using a fluence of 4.2 mJ/cm2 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.
@article{arxiv.2205.13212,
title = {Tracking the surface atomic motion in a coherent phonon oscillation},
author = {Davide Curcio and Klara Volckaert and Dmytro Kutnyakhov and Steinn Ymir Agustsson and Kevin Bühlmann and Federico Pressacco and Michael Heber and Siarhei Dziarzhytski and Yves Acremann and Jure Demsar and Wilfried Wurth and Charlotte E. Sanders and Philip Hofmann},
journal= {arXiv preprint arXiv:2205.13212},
year = {2022}
}