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

Abstract

Slow highly charged ions (HCIs) exhibit interesting phenomena when interacting with a solid. For example, a single ion can induce nanometer-sized deformations on a surface [1] and lead to the emis- sion of more than 100 electrons [2]. These effects result from the ion’ s large potential energy, ranging from tens to hundreds of keV [3], which is released within femtoseconds [4] upon impact. However, to harness the full potential of HCIs for technological applications, such as surface modification and analysis, a deeper understanding of the in- teraction processes is essential. Transmission experiments with free- standing 2D materials provide an ideal platform for such studies, as they allow us to isolate the surface-specific contributions in the HCI- solid interaction and provide insights into the timescales of the neu- tralization and energy deposition processes due to a limited interac- tion time with the solid material. Here, we focus on the emission of electrons as one of the main mechanisms for potential energy release. In particular, we want to tackle the question: from which side of the target are the electrons emitted? To find the answer, we measured the electron emission induced by slow highly charged xenon ions (up to Xe35+) transmitting through a freestanding single layer of graphene, for both the entrance and exit side, respectively. By detecting the emitted electrons in coincidence with the transmitted ion, we can relate the electron yield to specific ion impact parameters, energy loss and charge exchange. Preliminary results show that the electron yields are similar on both the entrance and exit side. If we look at the dependence of the electron yield on the number of electrons which the ion captures during the interaction, we find opposite trends on entrance and exit side. These measurements help us to unravel the complexity in HCI–surface collisions and to pinpoint the energy deposition process, timing and location. References: [1] Schwestka, J. et al., ASC Nano (2020), 14 1936-0851 [2] Schwestka, J. et al., J. Phys. Chem. Lett (2019), 10 4805−4811 [3] Gillaspy, J.D., J. Phys. B: At. Mol. Opt. Phys. (2001), 34 R93 [4] Niggas, A. et al., Commun. Phys. (2021), 4 2399-3650