Calculation of cross sections for2s−2pexcitation of one-electron ions byHe2+andHe0

Cross sections for $2s-2p$ excitation of one-electron ions scattering from ${\mathrm{He}}^{2+}$ and ${\mathrm{He}}^0$ are evaluated in the planewave Born approximation. The total cross section diverges logarithmically as the $2s-2p$ binding energy difference $\Delta E$ goes to zero for excitation by ${\mathrm{He}}^{2+}$ due to contributions from small momentum transfers. In contrast for ${\mathrm{He}}^0$ as $\Delta E$ goes to zero the total cross section is finite since the small-momentum-transfer contribution is damped corresponding to screening of the nucleus by the helium electrons at large distances. Calculations are presented for $2s-2p$ excitation of F and Si scattering from ${\mathrm{He}}^{2+}$ and ${\mathrm{He}}^0$ at velocities of several MeV/amu. For F and Si there is about an order of magnitude difference between scattering from ${\mathrm{He}}^{2+}$ and from ${\mathrm{He}}^0$, suggesting that electron screening is important here. However, our results are not in consistent agreement with recent observations.