A recent theoretical paper [Colgan et al., J. Phys. B 44, 175205 (2011)] has reopened the problem of C${}^{6+}$ ionization of He at 100 MeV/amu. The issue concerns ionization in the plane perpendicular to the momentum transfer $\mathbf{q}$ for the case where the ejected electron has an energy of 6.5 eV and $q=0.75$ a.u. Here, even after deconvolution, experiment finds two peaks near 90${}^{\circ }$ and 270${}^{\circ }$, contrary to earlier theoretical works that had predicted dips. Now, Colgan et al. also find peaks. We have reinvestigated the problem using the second Born approximation whose results are supported by a nonperturbative impact-parameter coupled-pseudostate approximation. Dips are again predicted. By comparing C${}^{6+}$ impact with that of its antiparticle ${\overline{\mathrm{C}}}^{6-}$ a “mirror-image” situation is observed in which the dips are converted into peaks, i.e., positively (negatively) charged projectiles give dips (peaks) near 90${}^{\circ }$ and 270${}^{\circ }$. A physical interpretation of this result is suggested. Applying exactly the same second Born approximation to $e^{\pm }$ impact ionization of He at 1 keV under similar dynamical conditions to C${}^{6+}$, we see exactly the same patterns, but now the results for $e^{-}$, showing peaks near 90${}^{\circ }$ and 270${}^{\circ }$ in the plane perpendicular to $\mathbf{q}$, are in quite good agreement with experiment in this and other geometries. This is strong support for the predictions of the second Born approximation for C${}^{6+}$. A paper by Egodapitiya et al. [Phys. Rev. Lett. 106, 153202 (2011)] warns of coherence problems with heavy-particle beams. It is suggested that this, together with experimental resolutions, may explain the experimental data for C${}^{6+}$.

• Received 11 May 2012

DOI:https://doi.org/10.1103/PhysRevA.85.062701