Speaker
Description
Secondary electron emission is fundamental to electron-surface interactions, underpinning technologies from microscopy to semiconductor processing. Yet the microscopic pathways driving low-energy emission (<20 eV) remain elusive. In particular, secondary electron spectra are often simply described as broad and featureless as a result of multiple scattering, concealing the underlying physics. Coincidence
spectroscopy can help to overcome this challenge by detecting electron pairs from the same scattering event, e.g. a scattered primary electron after interaction with the material and a secondary electron. This approach allows us to study otherwise hidden spectral features in the secondary electron spectrum.
We have applied electron pair emission spectroscopy to trace
the evolution of secondary emission across the dimensional transition from bulk graphite through bilayer to monolayer graphene. Remarkably, we uncover layer-dependent features: the prominent 3.3 eV resonance (often termed the X peak [1]) in graphite, another peak at 7.7 eV dominating the bilayer graphene spectrum, and no distinct spectral feature in monolayer
graphene. These signatures defy a simple explanation in terms of just the surface density of states alone. Instead, density functional theory calculations reveal Feshbach-type resonances of quasi-bound above-vacuum states — so-called doorway states — that couple material excitations to the continuum [2,3].
In this contribution we discuss how low-energy electron emission becomes a sensitive probe of material-specific electronic structure, with coincidence spectroscopy serving as the key that unlocks its full diagnostic potential.
[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)