Angular Distribution of Electrons emitted from Quasi-Freestanding Bilayer Graphene due to Impact of Slow Highly Charged Xenon Ions

Abstract

When slow highly charged ions (HCIs) impact a solid surface their potential energy (the sum of binding energies of all removed electrons) is re- leased within the topmost atomic layers of the material on femtosecond timescales [1, 2]. One dissipation mechanism of this deposited energy is secondary electron emission, which can lead to electron yields ranging from tens to more than a hundred electrons emitted per impacting ion. While total electron yields have been extensively studied, far less is known about the angular and energy distributions of the low-energy electrons (< 20eV), which even dominate the energy spectrum [3, 4]. These electrons are thought to result from electron cascades and collective relaxation processes such as plasmon excitation and decay. Yet, we recently demonstrated that high electron yields also occur for freestand- ing low-dimensional materials like monolayer graphene, even though inelastic electron cas- cades are less likely to occur there [2]. To study these low-energy electrons in more de- tail, we performed angle-resolved ion-induced electron emission spectroscopy (ARIIEES). We mounted a small, transportable electron beam ion source (EBIS-C1 from D.I.S. Germany) [5] at the ASPHERE III setup in DESY’s P04 beam- line. Using this source, we produced xenon ions inchargestatesfromq = 1toq = 20. The ions were directed onto quasi-freestanding bi- layer graphene under an incidence angle of 60 with respect to the surface normal. The resulting secondary electron emission was analysed with a hemispherical electron analyser, providing both energy distributions and angular patterns of the emitted electrons. Our measurements demonstrate that the angular distribution of emitted electrons depends on the incident charge state: singly charged projectiles exhibit nearly cosine-like distributions, whereas higher charge states produce enhanced emission at oblique angles. In particular, we observe a de- crease of emission along the surface normal. This behaviour can be explained by space charge effects: larger ion charge states leads to higher electron yields, i.e., more electrons leave the sample from the nanometer-sized impact area within only a few femtoseconds [2]. Electrons emitted along the surface normal repel each other more strongly, leading to a reduced probability of observation under normal emission direction. In this contribution, we will discuss these recent results, highlighting ARIIEES as a powerful tool to probe ultrafast charge dynamics at surfaces. References [1] E. Gruber et al. Nat. Commun. 7 13948 (2016) [2] A. Niggas et al. Phys. Rev. Lett. 129 086802 (2022) [3] P. A. Zeijlmans van Emmichoven et al. Phys. Rev. A 47 3998 (1993) [4] R. A. Baragiola et al. Phys. Rev. Lett. 76 2547 (1996) [5] D. Thima et al. J. Phys. B 57 165202 (2024)