Title: Ultrafast low-energy photoelectron diffraction for the study of surface-adsorbate interactions with 100 femtosecond temporal resolution

Abstract: An ultrafast photoemission-based low-energy electron diffraction experiment with monolayer surface sensitivity is presented. In a first experiment on tin-phthalocyanine adsorbed on graphite, we demonstrate a time resolution of approx. 100 fs. Analysis of the transient photoelectron diffraction signal indicates a heating of the adsorbate layer on a time scale of a few ps, suggesting coupling to phononic degrees of freedom of the substrate as the primary energy transfer channel for the vibrational excitation of the adsorbate layer. Remarkably, the transient photoelectron diffraction signal not only provides direct information about the structural dynamics of the adsorbate, but also about the charge carrier dynamics of the substrate. The presented concept combined with momentum microscopy could become a versatile tool for the comprehensive investigation of the coupled charge and vibrational dynamics of relevance for ultrafast surface processes.