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Elastic collision characteristics as a function of the relative speed of helium atoms with protons (red line), He+ ions (blue), hydrogen atoms (green), and helium atoms (purple): (a) integral cross section, (b) mean scattering angle, (c) mean parallel speed change per collision. The dashed line in panel (a) shows a parabolic fit in log–log scale for collision energies below 100 eV (see Appendix B)
