  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.
