Collisions of highly charged ions with surfaces: Does electron emission occur above the surface or below?

Abstract

Just as fast-moving particles lose kinetic energy through interactions with electrons in matter, slow highly charged ions dissipate their potential energy via electronic interactions. As the ion captures and stabilizes electrons from the target, its potential energy excites the target electronic system or is released through the emission of numerous low-energy electrons [1]. These processes occur on ultrafast timescales (femtoseconds) and are confined to the topmost atomic layers [2], making them difficult to study directly. Two-dimensional materials offer a unique opportunity to study these interactions and spatially resolve where electron emission originates. In this contribution, we present preliminary results on electron yields measured separately on both sides of a freestanding single layer of graphene (SLG) irradiated with highly charged xenon ions. By detecting the transmitted ion in coincidence with emitted electrons, we correlate electron emission with the kinetic and potential energy loss of the ion and the local surface cleanliness at the impact point. As shown in Fig. 1, we observe remarkably high electron yields of up to ~60 electrons per ion on each side. Comparing electron yields with the degree of neutralization of the ion reveals different trends on entrance and exit side, suggesting distinct emission mechanisms above and below the surface. [1] J. Schwestka et al; Charge-Exchange-Driven Low-Energy Electron Splash Induced by Heavy Ion Impact on Condensed Matter; The Journal of Physical Chemistry Letters, 2019 [2] A. Niggas et al; Peeling graphite layer by layer reveals the charge exchange dynamics of ions inside a solid; Communications Physics, 2021