  Hot carrier spatial and momentum distributions in nanoplasmonic systems
depend sensitively on the optical excitation parameters and nanoscale geometry,
which therefore determine the efficiency and functionality of plasmon-enhanced
catalysts, photovoltaics, and nanocathodes. A growing appreciation over the
past decade for the distinction between volume- and surface-mediated
photoexcitation and electron emission from such systems has underscored the
need for direct mechanistic insight and quantification of these two processes.
Toward this end, we use angle-resolved photoelectron velocity mapping to
directly distinguish volume and surface contributions to nanoplasmonic hot
electron emission from gold nanorods as a function of aspect ratio, down to the
spherical limit. Nanorods excited along their longitudinal surface plasmon axis
exhibit surprising transverse photoemission distributions due to the dominant
volume excitation mechanisms, as reproduced via ballistic Monte Carlo
modelling. We further demonstrate a screening-induced transition from volume
(transverse) to surface (longitudinal) photoemission with red detuning of the
excitation laser and determine the relative cross-sections of the two
mechanisms via combined volume and surface multiphoton photoemission modelling.
Based on these results, we are able to identify geometry- and material-specific
contributions to the photoemission cross-sections and offer general principles
for designing nanoplasmonic systems to control hot electron excitation and
emission distributions.
