Does electron emission in highly charged ion collisions with surfaces occur above the surface or below?

Abstract

Due to its large potential energy, a single slow highly charged ion (HCI) can induce the emis- sion of over 100 low-energy electrons from a solid surface [1]. However, experimental obser- vation of the underlying electronic processes is challenging, as they occur within just a few femtoseconds [2]. As a result, even after decades of research on HCI-surface interactions, the ex- act origin of these numerous emitted electrons – whether from above or below the surface – re- mains an open question. To address this, we minimize the sample thick- ness to a single atomic layer and measure elec- tron emission above (entrance side) and below (exit side) the two-dimensional target when irra- diated by HCIs. Using a coincidence detection technique, we simultaneously record ion param- eters (exit charge state, scattering angle and time-of-flight) and the electron yields from both entrance and exit side separately. Here we present preliminary results on electron emission from entrance and exit side of a free- standing single-layer graphene (SLG) sample ir- radiated by 129Xe30+ ions. We observe a entrance side clean SLG exit side ion velocity significant number of electrons on both sides of the sample. On the entrance side, the electron yield correlates with the degree of neutralization of the ion during transmission: the more elec- trons the ion captures and stabilizes, the higher the electron yield. In contrast, exit-side emission from clean SLG appears largely independent of ion neutralization. These patterns persist across varying ion veloci- ties, as shown in Fig. 1, indicating distinct emis- sion mechanisms on the entrance and exit sides. Further analysis may shed light on the dynamics of HCI-surface interactions, particularly the tim- ing and location of potential energy deposition. Moreover, exit-side electron emission could serve as a sensitive probe for the quality and thickness of two-dimensional materials. References [1] J. Schwestka et al. J. Phys. Chem. Lett. 10 4805−4811 (2019) [2] A. Niggas et al. Commun. Phys. 4 180 (2021)