(a) Conceptual representation of the energy-space-resolved PINEM methodology. Rather than recording an energy-filtered 2D spatial map of the transmitted electrons (images on the left), or dispersing the electrons only in energy (spectrum on top), this method retains the spatial electron distribution along the vertical axis, while also dispersing the electrons according to their energy along the horizontal axis. Combined with the PINEM effect, this results in the vertical spatial variation of the photoinduced SPP field being duplicated at equidistantly spaced energy quanta, with an intensity envelope and energy resolution determined by the PINEM interaction strength and the ZLP-width, respectively. The vertical spatial variation here (solid white trace) corresponds to a selected part (white shaded area) of the simulated photoinduced field (|Ez| in the plane 10 nm below the wire) of an isolated nanowire (black shaded rectangle, 4.6 μm length, ≃61 nm radius, 800 nm excitation, ϕ = 0°, m=13), indicated on the right. Electron counts in both images are plotted on the same linear scale. (b) The experimentally obtained energy-space image, taken on a selected section of a photoexcited nanowire (4.6 μm length, ≃61 nm radius, 800 nm excitation, ϕ = 0°, Δt=0 ps) is displayed together with a horizontal cut (along the energy axis, white dashed line), showing the quantized energy dependence of the interferometric spatial distribution of the SPP field. A Gaussian-fitted ZLP peak was subtracted and the intensity (electron counts) is mapped on a logarithmic scale in both the image and the spectrum to enhance the contrast. (c) A vertical cross-section at the energy corresponding to the net exchange of five photon quanta (grey dashed line in b) is shown, depicting the spatial distribution of the plasmonic field with its characteristic interference fringes.