Impact-parameter dependence of electronic energy loss and straggling of incident bare ions on H and He atoms by using the coupled-channel method

A first-principles calculation based on an expansion of the time-dependent electronic wave function in terms of atomic orbitals (coupled-channel method) has been applied to evaluate the impact-parameter dependence of the electronic energy loss and the fluctuation in energy loss of swift ions colliding on H and He atoms at energies of 10 to 500 keV/amu. The results have been compared with experimental data as well as with other existing models, e.g., the local-density approximation in an electron-gas target, the harmonic-oscillator target treatment, and the first-order plane-wave-Born approximation. Our results show a nearly exponential shape of the mean electronic energy loss for small impact parameters, in contrast to the Gaussian shapes obtained by Mikkelsen and Sigmund [Nucl. Instrum. Methods B 27, 266 (1987)].