Several years ago a surprising symmetry breaking, strictly demanded by first-order theories, was reported in measured fully differential cross sections (FDCS) for single ionization by ion impact even for very small perturbation parameters η (projectile charge to speed ratio) [1]. For about a decade, the data could not even qualitatively be reproduced by any fully quantum-mechanical calculation. Only about two years ago a possible explanation for these puzzling discrepancies was offered [2]: in the case of ionization of H_2 by 75 keV p impact differences in the double differential cross sections (DDCS) for fixed projectile energy loss as a function of scattering angle were found depending on the transverse projectile coherence length, i.e. the transverse width of the projectile wave packet. It was argued that coherence effects could also be important in the FDCS for atomic targets. However, one difficulty in the analysis of these data is that coherence effects are not easy to unambiguously distinguish from projectile angular resolution effects because both depend on the projectile beam profile.
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