Anna Niggas : Unraveling Secondary Electron Emission Induced by Electron and Ion Impact with Coincidence Spectroscopy

Secondary electron emission induced by low-energy electron impact plays a central role in electron-surface interactions and in many surface-sensitive techniques. However, the underlying mechanisms responsible for electron emission are often difficult to identify because conventional measurement techniques average over many different excitation pathways. Coincidence spectroscopy provides a powerful tool to disentangle these processes by detecting correlated particles originating from the same scattering event. In electron–electron coincidence experiments, we detect pairs consisting of a scattered primary electron and a secondary electron emitted from the surface due to the interaction. This approach provides direct access to the energy transfer during the scattering process and allows individual excitation channels to be isolated. For example, the previously unexplained prominent 3.3 eV feature observed for graphite, often termed the “X-peak” [1], was recently explained through coincidence spectroscopy as a two-step process involving plasmon excitation followed by hybridisation of interlayer states [2]. Building on this approach, we investigated how low-energy electron emission evolves as graphite is reduced from a bulk layered system to the truly two-dimensional limit of graphene. Coincidence measurements on highly oriented pyrolytic graphite (HOPG) as well as quasi-freestanding single-layer and bilayer graphene reveal clear layer-numberdependent features: while HOPG shows the strong X-peak resonance at 3.3 eV, bilayer graphene exhibits a weaker feature at 7.7 eV but no pronounced resonance appears for the monolayer system. A key concept emerging from these results is the role of doorway states: quasi-bound above-vacuum states that couple bulk excitations to free-electron states in the vacuum and thereby enhance emission at specific energies [3]. In the final part of the talk, I will briefly discuss a complementary coincidence approach using slow highly charged ions transmitted through atomically thin materials [4,5]. By correlating the charge exchange and scattering angle of the outgoing ions with the number of emitted electrons, we can follow how the large potential energy of the projectile is released and deposited into the electronic system of the material. Together, these examples illustrate how coincidence spectroscopy provides a unique window into the microscopic excitation and emission processes at surfaces.

References :
[1] H. Yamane et al. Phys. Rev. B 64 113407 (2001)
[2] W.S.M. Werner et al. Phys. Rev. Lett. 125 196603 (2020)
[3] A. Niggas et al. Phys. Rev. Lett. 135 166401 (2025)
[4] A. Niggas et al. Commun. Phys. 4 180 (2021)
[5] A. Niggas, J. Schwestka et al. Phys. Rev. Lett. 129 086802 (2022)