Disentangling primary from secondary electrons in ion-surface interactions

Abstract

Possible applications of slow highly charged ions (HCIs), for example in novel surface analy- sis techniques [1] and defect engineering, require a deeper understanding of the underlying neutral- ization and energy-deposition dynamics in both projectile and target. One sensitive observable of these processes is emission of electrons. When a slow HCI approaches a surface, the interaction is dominated by its internal potential energy, arising from the prior removal of multi- ple electrons. This energy can reach tens to hun- dreds of keV [1]. Upon impact on a solid, it is released on a femtosecond timescale [2] and de- posited very locally near the surface [3]. The en- suing charge exchange between target and HCI is accompanied by the emission of numerous low-energy electrons [4]. Using bulk targets, the number of primary electrons generated during ion impact cannot be measured unambiguously but contains a back- ground contribution from subsurface collision cascades and secondary electron emission. To in- crease the fraction of primary electrons, we use freestanding single-layer graphene (SLG) as an experimental model system for the surface of highly oriented pyrolytic graphite (HOPG). Its monoatomic thickness strongly suppresses mul- tiple-electron scattering in the target, while al- lowing for detection of electrons emitted on the back side of the target. We perform coincidence spectroscopy of Xe30+ ions transmitted through freestanding SLG to- gether with the electrons emitted from both sides of the graphene [6]. To clarify the origin of the measured electrons, we compare the data to simulations based on the classical-over-bar- rier model [5] for charge exchange and deexci- tation processes. ________________________________________________________ * E-mail: vojtech@iap.tuwien.ac.at Measurements on HOPG under identical experi- mental conditions further show that the HOPG yield exceeds the entrance-side yield from SLG, while remaining well below the total SLG yield. This indicates that, for bulk targets, a substantial fraction of the primary electrons is reabsorbed by the target, where it contributes to the electronic response and internal excitation of the solid.