# 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  
Cross-linking of a self-assembled monolayer of organic molecules by low energy electron irradiation can lead to a closed and mechanically stable membrane, which can be released from a substrate and used as a freestanding sheet [47]. Using 1,1\({}^{\prime}\)-Biphenyl-4-thiol as a molecular precursor results finally in a 1 nm thick carbon nanomembrane [48]. The sheet thickness is small enough to enable highly charged ion transmission such that the transmitted ions have not yet reached their charge state equilibrium [19]. In this way, we are able to address pre-equilibrium charge exchange and stopping directly. Figure 4a shows a spectrum of charge states obtained with an electrostatic analyzer in forward direction and an acceptance angle of \(1.6^{\circ}\) (see Figure 4b). Xe\({}^{20+}\) ions with a kinetic energy of 90 keV were used. Each peak can be assigned to an exit charge state. The position of the peaks is given by the ion energy loss and the width of each peak is determined by the ion energy straggling. The large peak at \(Q=20\) results from ions, which pass uncovered areas from the sample. These areas cannot be avoided, because 2D materials cover a supporting TEM grid only partially and rip apart due to mechanical stress. However, the \"primary peak\" at \(Q=20\) allows an energy calibration inherent to every spectrum, because these ions did not exhibit energy loss. The spectrum in Figure 4a shows even more peaks at lower charge states with a decreasing height from \(Q=19\) to \(Q=8\) and and increasing abundance from \(Q=8\) to \(Q=4\). With an electrostatic analyzer, we are limited to a certain maximal voltage, which, in turn, limits the maximal observable charge exchange. Further, ions transmitted as neutral atoms cannot directly be observed. The decreasing and increasing intensities of exit charge states are in fact two distinct charge state distributions, as shown by angle-resolved measurements recently [19]. Using a different detection technique, i.e., a multi-channel plate with a delay-line detector (DLD), enables a two-dimensional position determination of transmitted ions (see Figure 4e) [24]. Limiting the scattering angle in one direction (vertical) with a slit aperture and using a set of deflection plates, we can resolve both, charge exchange and scattering angle, at the same time for all transmitted charge states (including neutral atoms) in parallel. Figure 4c shows a charge exchange spectrum from 180 keV Xe\({}^{20+}\) with this detection technique. The ion parameters are, and thus the spectrum is, comparable to the data shown in Figure 4a. Along the \(y\)-axis, lines appear of different intensities (color) resulting from different exit charge states. The scattering angle is shown along the \(x\)-axis. The DLD can be moved along the horizontal axis up to \(10^{\circ}\) to cover a wide range of scattering angles with subsequent measurements (not shown here). Integrating the exit charge state intensities in an angular range of \(\pm 0.25^{\circ}\) results in the exit charge state spectrum shown in Figure 4d. The spectrum is similar to the data presented in Figure 4a, whereas it also includes charge states \(Q=0\)-\(3\). It shows that the second, low exit charge state distribution indeed peaks at \(Q=2\). Interestingly, the angular dependence of thecharge exchange can here be observed directly with a resolution of about 0.01\({}^{\circ}\), not possible with the electrostatic detector set-up. Low exit charge states i.e., \(Q=0\)-6 are scattered into a broad angular range, whereas the maximum scattering angle for Xe on \(C\) is given by momentum conservation and amounts to about 5.3\({}^{\circ}\). This observation was already made earlier with angle-resolved measurements utilizing the electrostatic detector set-up and led to the conclusion, that ions in low exit charge states are scattered at small impact parameters and are subsequently deflected under larger angles [19].