Electronic Excitations of Slow Ions in a Free Electron Gas Metal: Evidence for Charge Exchange Effects

Abstract

Electronic energy loss of light ions transmitted through nanometer films of Al has been studied at very low ion velocities. For hydrogen, the electronic stopping power S is found to be perfectly proportional to velocity, as expected for a free electron gas. For He, the same is anticipated, but S shows a transition between two distinct regimes, in both of which S is velocity proportional-however, with remarkably different slopes. This finding can be explained as a consequence of charge exchange in close encounters between He and Al atoms, which represents an additional energy loss channel.

Accurate atomistic first-principles calculations of electronic stopping

“…Finally, in Fig. 5(a) we compare our results for channeling as well as off-channeling projectiles to experimental 53 and other theoretical 30,54 results, finding very good agreement with measurements. 53 The experimental electronic stopping is only slightly larger: the curve corresponding to the channeling projectiles is within approximately 12 % (or better) of the experimental result for velocities between 0.1 and 0.45 at.…”

“…We note that the experimental data in Ref. 53 originates indeed from different experiments, and we suggest that those may correspond to different channeling geometries. Further experimental and theoretical work is necessary to clarify this point.…”

“…The comparison with perturbative theory is better justified for the hydrogen projectile than for other projectiles; and the stopping geometry as well as convergence aspects of the problem can be characterized with more exhaustive detail. We compare our results to the empirically-fitted SRIM results, as an accurate summary and average of numerous experiments, 11 as well as other experimental 53 and theoretical works. 22,30,54 Figure 3 shows our calculated results for channeling as well as off-channeling projectiles in different charge states and for different valence-electron configurations.…”

Electronic Stopping of Slow Protons in Transition and Rare Earth Metals: Breakdown of the Free Electron Gas Concept

“…While at v < 0.2 a.u., the nonlinear FEG model [10] successfully describes the stopping ratio Pt/Au  1.7 by use of effective FEG densities [17], for Gd and Ta, the FEG concept fails: by use of QH (rs) from theory [10,37], the slopes of the experimental data can be converted to For Gd, the value for rs,expt is almost identical to that of Al [40]. Hence, SGd  SAl, in accordance with the fact that Nval = 3 for both Al and Gd.…”

Electronic Stopping of Slow Protons in Oxides: Scaling Properties

“…The breakdown of the additivity rule can be seen in Fig. 2a, where the low velocity SCS of selected oxides (ZnO, VO2, SiO2, HfO2, Al2O3, Ta2O5) are presented together with Bragg's rule predictions using data from [36,41,42,43]:…”