Angle-resolved spectroscopy of secondary electrons from surfaces emitted by highly charged ion impact

Abstract

Secondary electron emission is one of the main energy dissipation channels of a highly charged ion impact on a material surface. While the electron yield, i.e. the number of electrons emitted per impacting ion, has been studied extensively for different ion velocities and charge states [1,2], literature on angle- and energy-resolved measurements of low-energy secondary electrons is scarce [3,4]. In particular, a mo- mentum space image of emitted electrons through angle-resolved de- tection would clarify if the electrons are emitted isotropically from the ion itself or anisotropically from the solid surface through processes such as the interatomic Coulombic decay (ICD) [5]. We installed a small, portable electron beam ion source [6] at the AS- PHERE III setup at DESY, which is equipped with a hemispherical energy analyser, allowing us to perform angle-resolved ion-induced electron emission spectroscopy (ARIIEES). Our results show that the angular emission pattern depends on the electron yield: For singly- charged ions, the electron emission distribution resembles a cosine dis- tribution, which flattens with increasing charge state. In other words, higher ion charge states result in increased total secondary electron yields where emission to larger angles with respect to the surface nor- mal is favoured. A possible explanation of this result is that space charge effects lead to the repulsion of electrons starting along the sur- face normal, which would only become relevant at high incident ion charge states where the electron yield increases. In this contribution, I will discuss these recent results and put them into context to show how this distortion of the electron emission spectrum may impact other ex- perimental techniques. <br> [1] H. Kurz et al., Phys. Rev. A 49 4693 (1994) [2] A. Niggas, J. Schwestka et al., Phys. Rev. Lett. 129 086802 (2022). [3] P. A. Zeijlmans van Emmichoven et al., Phys. Rev. A 47 3998 (1993). [4] H. Eder et al., Surf. Sci. 472 195 (2001). [5] R.A. Wilhelm et al., Phys. Rev. Lett. 119 103401 (2017) [6] D. Thima, A. Niggas et al., J. Phys. B: At. Mol. Opt. Phys. 57 165202 (2024).