Electronic stopping cross sections of 50-300-keV He and Li ions

For 50-300-keV ${\mathrm{He}}^{+}$ and ${}_{}{}^7{}_{}{}^{}\mathrm{Li}_{}^{+}{}_{}{}^{}$ in C, Al, Cu, Ag, and Au, the applicability of the simplified stopping function is investigated. Using the same targets for ${\mathrm{He}}^{+}$ and ${\mathrm{Li}}^{+}$ the electronic stopping power $S_e$ in 400-800-Å foils is measured by means of a magnetic spectrometer. The resulting stopping fractions $\frac{S_e\left(\mathrm{Li}\right)}{S_e\left(\mathrm{He}\right)}$ can be evaluated to an error of 2%. At energies above 20 keV/amu they are found to be independent of the target material. Applied to our experimental data, the simplified stopping function is shown to be a good approximation in connection with the fits of Andersen-Ziegler for $S_e$ (H) and the stopping ratios.