# Neutralization Dynamics of Slow Highly Charged Ions in 2D Materials
## 2 Experimental Results on Highly Charged Ion Transmission
### Our Work
#### 2.2.1 Carbon Nanomembranes
Ions in the first, high exit charge state distribution are scattered within an angular range of about \(\pm\)0.15\({}^{\circ}\) with a charge state dependent scattering pattern. Charge states of \(Q=14\)-19 are transmitted directly into forward direction indicating no angular deflection (angular spread is comparable to the \"primary peak\" at \(Q=20\)) or, in turn, they are scattering with large impact parameters (\(\gtrsim\)1.5 A [19]). Now, ions at charge states \(Q=7\)-13 are transmitted in angles of 0.05-0.15 \({}^{\circ}\), i.e., not in straight forward direction. From the classical scattering integral  
\[\theta_{cms}(p,E_{cms})=\pi-2\int_{R_{min}}^{\infty}\mathrm{d}R\frac{p}{R^{2} \sqrt{1-\frac{V(R)}{E_{cms}}-\frac{p^{2}}{R^{2}}}} \tag{1}\]  
we can calculate the impact parameter for these particular scattering angles and thus derive the impact parameter dependence of charge exchange [21]. In Equation (1), the scattering angle \(\theta\) is given in the center-of-mass (cms) system. \(R_{min}\) is the distance of closest approach of the projectile and the target atom, \(p\) the impact parameter, \(R\) the interatomic distance, \(E_{cms}\) the ion kinetic energy in the cms and \(V(R)\) is the scattering potential. The scattering potential \(V(R)\) is charge state dependent. Thus, for each incident charge state (and to some extent each charge exchange value), the scattering integral must be evaluated [21; 49]. However, the evaluation and the comparison with the COB-model for the high exit charge states (\(Q>7\)) is beyond the scope of this paper. At this point, we see for CNMs two exit charge state distributions, one at high and one at low exit charge states. Both distributions also show distinctly different angular scattering patterns, whereas high exit charge states are observed mainly under straight forward direction (\(\pm\)0.15\({}^{\circ}\)) and low exit charge states at all angles within 0-5.3\({}^{\circ}\) (or larger angles resulting from multiple scattering). As the charge exchange in CNMs is large, i.e., HCIs are almost neutralized in 1 nm thick carbon membranes (for the low charge state distribution), a large amount of potential energy is deposited (see Figure 1). This potential energy deposition leads to the formation of large pores with diameters  
Figure 4: (Color online) (**a**) Charge state spectrum of 90 keV Xe\({}^{20+}\) ions transmitted through a 1 nm thick freestanding carbon nanomembrane. (**b**) Experimental scheme for (**a**) with an electrostatic analyzer as a detector with 1.6\({}^{\circ}\) acceptance angle. (**c**) 2D charge state spectrum of 180 keV Xe\({}^{20+}\) after transmission through a 1 nm thick carbon nanomembrane with different exit charge state positions marked by white lines and the scattering angle on the abscissa (please note the log-scale). The vertical position (\(y\)) can be converted into a charge state according to the deflection by an electric field indicated in (**e**). (**d**) Integrated charge state spectrum of (**c**) for scattering angles between \(-\)0.25\({}^{\circ}\) and +0.25\({}^{\circ}\). (**e**) Experimental scheme for (**c**) with a position sensitive MCP as a detector with 0.01\({}^{\circ}\) angular resolution. of several nanometers in CNMs (see Figure 5). A single HCI removes about 100-1000 atoms, which is a potential sputtering yield comparable to the case of 3D target surfaces (see Figure 3a) [18; 20]. The mean size of the pores can be adjusted by the incident charge state in a range of 0 (\(Q<22\)) to about 17 nm (40 keV Xe\({}^{40+}\)) in diameter. Even though the mechanism for pore formation is still not entirely clear, the insulating behaviour of CNMs makes them prone to HCI induced nanostructuring. Deposited positive charge and trapped electronic excitations at the impact site, as a result of the charge exchange, may not be screened fast enough before atoms from the material are set in motion. Thus, potential sputtering may occur as a result of a simple bond breaking with subsequent desorption.