  Plasmons can be excited during photoemission and produce spectral
photoelectron features that yield information on the nanoscale optical response
of the probed materials. However, these so-called plasmon satellites have so
far been observed only for planar surfaces, while their potential for the
characterization of nanostructures remains unexplored. Here, we theoretically
demonstrate that core-level photoemission from nanostructures can display
spectrally narrow plasmonic features, reaching relatively high probabilities
similar to the direct peak. Using a nonperturbative quantum-mechanical
framework, we find a dramatic effect of nanostructure morphology and
dimensionality as well as universal scaling laws for the plasmon-satellite
probabilities. In addition, we introduce a pump--probe scheme in which plasmons
are optically excited prior to photoemission, leading to plasmon losses and
gains in the photoemission spectra and granting us access to the ultrafast
dynamics of the sampled nanostructure. These results emphasize the potential of
plasmon satellites to explore multi-plasmon effects and ultrafast
electron--plasmon dynamics in metal-based nanoparticles and two-dimensional
nanoislands.
