Calculations on the slowing down of 0.4-4.0-MeV He ions in solids

The localization of atoms in solids leads, in principle, to changes in the electron distribution compared with the free-atom state. The effect of such changes on the calculation of the He-ion stopping cross section $S_e$ in solids has been investigated for an energy range 0.4-4.0 MeV. It is found that at low energies with free-atom wave functions, a large part of the calculated $S_e$ arises from regions at large distances from the target atom where the electron density is low compared to that in solids. The experimentally observed oscillations in $S_e$ as a function of target atomic number, which have been calculated from stopping-power theory by other authors, are also obtained in the present work with simple modifications of the electron density distributions. However, the detail of the structure of the stopping curves is altered. In addition, the use of an effective charge to account for electron capture by the He ion at low velocities, without a corresponding modification of the basic stopping-power theory to take into account the departure from point-charge behavior, leads to discrepancies from the experimental data.