Is electronic stopping of ions velocity-proportional in the velocity-proportional regime?

Abstract

We have studied systematic deviations of reported electronic stopping cross sections from velocity proportionality predicted by Lindhard and Scharff. Most pronounced is an apparent drop-off at beam energies below ∼ 0.01 MeV/u. In the frequently-used transmission technique, only a narrow cone of the beam is analysed in order to minimize the contribution from nuclear stopping. This has the consequence that a reduced instead of the full electronic energy loss is recorded. The magnitude of this effect has been computed on the basis of a recently developed extension of the scheme of Fastrup et al. to correct for nuclear stopping. In a detailed study of the CAr system based on our PASS code we find close agreement between the derived reduced electronic stopping cross section and the experimental result reported by Ormrod. Results are reported for eight ions on argon, for O in C and Cl in C and for Ni in Ag. For Ni in Ag as well as for one set of reported stopping cross sections for N in Ar a particularly sharp drop-off is shown to be caused by an inadequate nuclear-stopping correction. Our findings do not support Paul's assertion of a gas-solid difference in electronic stopping of low-energy heavy ions.

On the influence of uncertainties in scattering potentials on quantitative analysis using keV ions

“…Note, that impact parameters and the associated interaction distances of relevance, can even for identical ion energies differ significantly in different experimental approaches: In ion implantation, all kind of trajectories and consequently all impact parameters, contribute necessarily to the final modification of the target material. In ion-beam based analysis, in different experimental approaches such as backscattering and transmission experiments designed to measure thickness, very different impact parameter regimes can be expected to be of interest [28,29]. Intrinsically, as not all primary ions are contributing to the relevant signal, both approaches will to certain extend be selective to specific trajectories.…”