Momentum-Resolved Photoelectron Emission from Single-Crystalline Graphite: from Direct Ejection to Slow Cascade Electrons

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Abstract
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.

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