  In condensed matter, scattering processes determine the transport of charge
carriers. In case of heterostructures, interfaces determine many dynamic
properties like charge transfer and transport and spin current dynamics. Here
we discuss optically excited electron dynamics and their propagation across a
lattice-matched, metal-metal interface of single crystal quality. Using
femtosecond time-resolved linear photoelectron spectroscopy upon optically
pumping different constituents of the heterostructure we establish a technique
which probes the electron propagation and its energy relaxation simultaneously.
In our approach a near-infrared pump pulse excites electrons directly either in
the Au layer or in the Fe layer of epitaxial Au/Fe/MgO(001) heterostructures
while the transient photoemission spectrum is measured by an ultraviolet probe
pulse on the Au surface. Upon femtosecond laser excitation, we analyze the
relative changes in the electron distribution close to the Fermi energy and
assign characteristic features of the time-dependent electron distribution to
transport of hot and non-thermalized electrons from the Fe layer to the Au
surface and vice versa. From the measured transient electron distribution we
determine the excess energy which we compare with a calculation based on the
two-temperature model and takes diffusive electron transport into account. On
this basis we identify a transition from a super-diffusive to a diffusive
transport regime to occur for a Au layer thickness of 20-30~nm.
