  Understanding the differences between photon-induced and plasmon-induced hot
electrons is essential for the construction of devices for plasmonic energy
conversion. The mechanism of the plasmonic enhancement in photochemistry,
photocatalysis, and light-harvesting and especially the role of hot carriers is
still heavily discussed. The question remains, if plasmon-induced and
photon-induced hot carriers are fundamentally different, or if plasmonic
enhancement is only an effect of field concentration producing these carriers
in greater numbers. For the bulk plasmon resonance, a fundamental difference is
known, yet for the technologically important surface plasmons this is far from
being settled. The direct imaging of surface plasmon-induced hot carriers could
provide essential insight, but the separation of the influence of driving
laser, field-enhancement, and fundamental plasmon decay has proven to be
difficult. Here, we present an approach using a two-color femtosecond
pump-probe scheme in time-resolved 2-photon-photoemission (tr-2PPE), supported
by a theoretical analysis of the light and plasmon energy flow. We separate the
energy and momentum distribution of the plasmon-induced hot electrons from the
one of photoexcited electrons by following the spatial evolution of
photoemitted electrons with energy-resolved Photoemission Electron Microscopy
(PEEM) and Momentum Microscopy during the propagation of a Surface Plasmon
Polariton (SPP) pulse along a gold surface. With this scheme, we realize a
direct experimental access to plasmon-induced hot electrons. We find a
plasmonic enhancement towards high excitation energies and small in-plane
momenta, which suggests a fundamentally different mechanism of hot electron
generation, as previously unknown for surface plasmons.
