Alessandra Bellissimo

High-resolution, momentum-resolved photoelectron emission microscopy is employed to investigate ejected electrons extending all the way from the directly emitted primary photoelectrons to the slow secondary electrons resulting from the ensuing collision cascade. Excitation with unpolarised radiation from a helium light source enables the valence band to be mapped across the entire first surface Brillouin zone. The measured quasi-particle bands show very good quantitative agreement with state-of-the-art density functional theory supplemented by many-body corrections within the GW approximation, establishing a reliable reference for the initial-state electronic structure. Extreme-ultraviolet light of controlled polarisation from a high-harmonic generation source is used to access high-lying conduction-band states. The polarisation-dependent momentum distributions of directly photoemitted electrons reveal the roles of initial-state symmetry and final conduction-band states. In the low-energy secondary-electron regime we map out, for the first time, the momentum-space distribution of the quasi-bound in-plane resonance in the conduction band which acts as a doorway state for secondary-electron emission through the X-peak. Six-fold-symmetric intensity patterns at characteristic kinetic energies above vacuum, independent of photon energy and light polarisation, indicate cascading secondary-electron emission through preferential conduction-band channels. These nearly non-dispersive features arise from hybridised, sp2-derived σ∗ conduction-band states. By disentangling contributions from the valence bands, conduction bands mediating direct photoemission, and low-energy resonances populated by the slowing-down electron cascade, our results provide unprecedented experimental access to the valence-band electronic structure and multiple emission channels of bulk graphite, relevant also to other layered quantum materials.

We investigate the secondary low-energy electron emission induced by inelastic electron scattering from graphene and layered materials thereof. By applying a coincidence detection of the primary scattered and the emitted secondary electron we unravel pronounced resonance features otherwise overshadowed by the largely structureless secondary electron energy distribution. Supported by density functional theory calculations we show that these structures are the signature of prominent resonances above the vacuum threshold which originate from interlayer states acting as a doorway state for electron emission. Remarkably, some of these doorway states open up only for samples with more than five layers.

Reflection electron energy loss spectroscopy (REELS) spectra were measured for three inorganic compounds (SiO ₂ , HfO ₂ and CaF ₂ ). VIS‐XUV optical constants were extracted from the measured REELS spectra after elimination of multiple inelastic scattering by deconvolution. The derived normalised single scattering loss distributions were fitted to a Drude‐type dielectric function. The resulting optical constants reasonably satisfy the Kramers–Kronig and ‐sum rules. Values for the energy band gap were also determined from the deconvoluted loss spectra. Special attention is paid to the extension of the optical constants onto the complex plane and its relation to the kinematics in an inelastic collision for a material with a given electronic structure. The analysis suggests differences in the elementary scattering kinematics in a REELS experiment between insulators and metals. Describing the dispersion of the loss features on the complex plane by the dispersion constant , we find that the dispersion constant in the analysis of REELS data should be treated as a fit parameter or, alternatively, use of a vanishing value for insulators and for conductors is recommended. For evaluating the inelastic mean‐free path (IMFP) from optical constants, should always be taken to be unity.

Reflection electron energy loss spectroscopy (REELS) spectra were measured for seven insulating organic compounds (DNA, Irganox 1010, Kapton, polyethylene [PE], poly(methyl methacrylate) [PMMA], polystyrene [PS] and polytetrafluoroethylene [PTFE]). Optical constants and energy band gaps were extracted from the measured REELS spectra after elimination of multiple electron scattering via a deconvolution and fitting the normalised single scattering energy loss spectra to Drude and Drude–Lindhard model dielectric functions, constrained by the Kramers–Kronig sum and f ‐sum rules. Satisfactory agreement is found for those optical constants for which literature data exists. For PTFE, the observed features in the optical data correspond to its electronic structure.

The signal generation mechanism of the scanning field-emission microscope has been investigated via model calculations combining deterministic trajectory calculations in the field surrounding the field-emission tip in vacuum, with Monte Carlo simulations of the electron transport inside the solid. This model gives rise to a two-dimensional electron cascade. Individual trajectories of detected backscattered electrons consist of repeated segments of travel in vacuum followed by a re-entry into the solid and re-emission into vacuum after being elastically or inelastically scattered. These so-called electron bouncing events also create secondary electrons at macroscopic distances away from the primary impact position. The signal reaching the detector is made up of elastically and inelastically backscattered primary electrons created near the impact position under the tip and those secondary electrons created far away from it.