  Electronic stopping of H and He ions in metals and insulators is analyzed at
velocities below 0.2 atomic units, i.e. below 1 keV for H and below 4 keV for
He. In metals, stopping of H ions is affected by d-electrons only when the
d-band extends up to the Fermi energy; for He ions, also d-bands well below the
Fermi energy contribute significantly to electronic stopping. In insulators,
the low threshold velocity for electronic stopping cannot be explained by
electron-hole pair excitation; charge exchange cycles, however, may govern the
threshold behavior of electronic stopping in ionic crystals.
