Ultrafast Low-Energy Photoelectron Diffraction for the Study of Surface-Adsorbate Interactions with 100-fs Temporal Resolution

A novel method of ultrafast electron diffraction for the study of structural dynamics at surfaces is presented. The probing electron pulse is generated in the immediate vicinity of the surface by photoemission of low-energy electrons from the sample substrate. This limits the propagation distance of the electron pulse before diffraction to a few nanometers and thus minimizes pulse broadening effects due to space charge and dispersion. The experimentally proven time resolution of ≈100 fs exceeds reported values of conventional ultrafast low-energy electron diffraction by a factor of 10.