  We show that the scattering interaction between a high energy electron and a
photon can be strongly enhanced by different types of localized plasmons in a
non-trivial way. The scattering interaction is predicted by an eigen-response
theory, numerically verified by finite-difference-time-domain simulation, and
experimentally verified by cathodoluminescence spectroscopy. We find that the
scattering interaction associated with dark plasmons can be as strong as that
of bright plasmons. Such a strong interaction may offer new opportunities to
improve single-plasmon detection and high-resolution characterization
techniques for high quality plasmonic materials.
