# Neutralization Dynamics of Slow Highly Charged Ions in 2D Materials
## 2 Experimental Results on Highly Charged Ion Transmission
### Earlier Work  
In a first attempt to determine the distance an ion travels in a solid until it reaches its equilibrium charge state, [PERSON] et al. [12] investigated the energy loss and final charge state distribution of slow (\(v=0.76\ v_{0}\)) Ar\({}^{Q_{\text{in}}+}\) ions (\(Q_{\text{in}}=8\), 12, and 16) after transmission through a freestanding 31 nm thick carbon foil. Within the experimental resolution, they could not find a dependence on the projectile's initial charge state and therefore concluded that the ions reach their charge state equilibrium after passing a few atomic layers only. Furthermore, transmission measurements through thinner carbon foils (10 nm and 5 nm) performed by [PERSON] et al. [41] could show that HCIs reach their equilibrium charge state after the transmission of a 10 nm thick foil but not after the passage of only a 5 nm thick carbon foil. Evaluating the average exit charge state in dependence on the projectile velocity (0.25-0.46 \(v_{0}\)), the charge equilibration time in the carbon foil could be estimated to be less than 7 fs. Since sequential filling of the ion's inner-shell vacancies would probably take much longer than the evaluated equilibration time of only a few fs, the authors concluded that multiple Auger transition cascades must proceed in parallel to facilitate such a rapid equilibration [41]. Besides transmission measurements, similar results were also obtained by performing scattering measurements with HCIs. [PERSON] et al. [42] investigated the final charge state distribution of multiply charged O\({}^{Q+}\) ions with \(Q=3\)-8 scattered under grazing angles of incidence from a gold surface and observed almost complete charge equilibration. With projectile trajectory calculations, they could show that the projectiles spend less than 30 fs within 2 A of the topmost Au layer. They concluded that already at the target interface strong screening effects have to be present which result in a significant upward shift of the projectile energy levels, enabling a direct capture of electrons into the L and M shell. Similar scattering processes were performed by [PERSON] et al. [43], who investigated the final charge state distribution of highly charged Ar\({}^{Q_{\text{in}}}\) ions (\(Q_{\text{in}}=4\)-17, \(v=0.15\)-0.62 \(v_{0}\)) scattered under grazing angles of incidence from a smooth highly oriented pyrolytic graphite surface. It was found that independent of their initial charge state, the scattered projectiles leave the surface virtually fully equilibrated. They explained the fast neutralization process and the X-ray spectra measured in coincidence with grazingly scattered ions by side feeding processes of electrons into the L and K shell. Apart from transmission measurements and scattering experiments, observation of fast electron ejection and measurements of the exit projectile charge state distribution after frontal collision of Xe\({}^{30+}\) ions with C\({}_{60}\) were performed by [PERSON] et al. [1]. This model system of a large atomic cluster with a small diameter limiting the ion's transmission time to less than 3.2 fs is already very close to a truly 2D solid surface. The assumption of greatly enhanced Auger rates was found to be necessary to interpret the observed fast electron ejection during the collision time. As long as neutralization is incomplete, charge state effects on the energy loss also become experimentally accessible. An enhancement of the average energy loss value as a function of the incident charge state was reported by [PERSON] et al. [44]. They performed transmission measurements with Xe\({}^{Q_{\text{in}}+}\) and Au\({}^{Q_{\text{in}}+}\) ions (\(v\approx 0.3\ v_{0}\)) in high charge states (\(Q_{\text{in}}\geq 40\)) through 10 nm thick carbon foils. In comparison to these measurements, in scattering measurements performed by [PERSON] et al. [45], a charge dependent energy loss could not be observed, which was, in turn, interpreted as indication for full neutralization of the projectile before reaching the distance of the closest approach.