Dynamic charge states of energetic He ions in silicon single crystals

The dynamic screening and the approach to charge-state equilibrium of energetic He ions passing through a silicon lattice is studied using the Rutherford backscattering technique. Measurements of breakthrough angles ${\psi }_M$ for planar channeled protons and He ions provide information on the dynamic equilibrium charge state of 250-keV He ions in silicon. The results can be described with a screened Molière-type potential, the He data requiring a shorter screening length than the proton data. Comparison of ${\mathrm{He}}^{+}$ and ${\mathrm{He}}^{2+}$ data shows that the time needed to establish charge-state equilibrium is not longer than approximately ${10}^{\mathrm{-}15}$ sec corresponding to a distance travelled of about 40 Å.