Electron microscopy techniques such as electron energy-loss spectroscopy (EELS) facilitate the spatiospectral characterization of plasmonic nanostructures. In this work, a time-dependent perspective is presented that significantly enhances the utility of EELS. In particular, this approach facilitates the analysis of the dynamics of plasmonic excitations that repeatedly interact with swift electrons in a STEM-EELS configuration. This includes the bulk plasmon mode, which can only be excited by penetrating electron beams, and the fundamental surface plasmon polariton modes propagating along the wire, which can be excited by both penetrating and aloof trajectories. In addition, the role of higher-order azimuthal surface plasmon polariton modes, often overlooked for very thin wires, is observed and analyzed in both the energy-loss spectrum and from the dynamical perspective. Such a complete understanding of the interaction of electrons and plasmonic excitations is key for the design
of efficient plasmonic sensors, the study of hot electron dynamics in metals, and applications in the context of electron quantum optics, where full control of the spatial and temporal characteristics of the fields at the nanometer and femtosecond scales is highly desirable.