Tracking the surface atomic motion in a coherent phonon oscillation

TL;DR

This study demonstrates ultrafast time-resolved photoelectron diffraction with soft X-ray pulses to track atomic motion during coherent phonon oscillations in a surface material, revealing atomic displacements on the order of 1 pm within 3 ps.

Contribution

It introduces a novel ultrafast diffraction technique combining free electron laser pulses with photoelectron diffraction to observe atomic motions in real time.

Findings

Successfully tracked near-surface atomic motion within 3 ps after excitation.

Measured coherent vibrational amplitude of about 1 pm in the first two interlayer spacings.

Achieved 140 fs temporal resolution in observing surface atomic dynamics.

Abstract

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.