  Efficient excitation and harvesting of hot carriers from nanoscale metals is
central to many emerging photochemical, photovoltaic, and ultrafast
optoelectronic applications. Yet direct experimental evidence of the relevant
femtosecond dynamics in ubiquitous tens-of-nanometer gold structures remains
lacking, despite the rich interplay between interfacial and internal plasmonic
fields, excitation distributions, and scattering processes. To explore the
effects of nanoscale structure on these dynamics, we employ a new technique for
simultaneous time-, angle-, and energy-resolved photoemission spectroscopy of
single plasmonic nanoparticles. Photoelectron velocity distributions reveal
bulk-like ballistic hot electron transport in nanorod and nanoshell geometries,
with no evidence of surface effects. Energy-resolved dynamics observed in the
~1-2 eV range and extrapolated to lower energies via kinetic Boltzmann theory
provide the first direct measurements of hot carrier lifetimes within nanoscale
gold. Remarkably, we find that particles with dimensions as small as 10 nm
serve as exemplary platforms for studying intrinsic metal dynamics.
