Ion-Induced Secondary Electron Emission from Graphite: The X-Peak observed upon Ion Irradiation

Abstract

Secondary electron emission is a key energy dissipation channel upon highly charged ion interactions with surfaces. While total electron yields are well studied, angle- and energy- resolved data on low-energy secondary electrons remain scarce. Here, we present such measurements using a compact electron beam ion source installed at the ASPHERE-III ARPES endstation at DESY’s PETRA III. We chose graphite as our target system — a material well-studied in the past in general, and in particular in recent years with focus on electron-induced processes. There, the prominent 3.3 eV resonance (often termed X peak [1]) could been identified as stepwise process of plasmon excitation [2] and decay through excitation of electrons into above- vacuum band structure states. Subsequently, through “doorway states”, these excited material states can couple to the continuum through Feshbach-type resonances of interlayer states [3]. Remarkably, we observe the same X peak when probing graphite with He, Ne, and Xe ions over a wide range of charge states and velocities. This demonstrates that the underlying excitation mechanism is not limited to electron impact but can also be driven by ion-induced energy deposition. The X peak intensity varies systematically with projectile mass, charge state, and velocity, revealing how the excitation mechanism depends on the projectile's energy deposition profile. These results demonstrate that ion-induced secondary electron spectroscopy provides a complementary probe into electronic excitation processes at surfaces and the surface band structure. [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) 16