Angular scattering in fast ion–atom electron transfer collisions: projectile wave diffraction and Thomas mechanisms

We report experimental angular differential cross sections for double-electron capture in He2 + + He collisions and single-electron capture in H+ + He collisions for the 1.3--12.5 MeV kinetic energy range. In all cases, the total cross sections are dominated by forward scattering peaks in d Delta *s/d Delta *W. The shapes and widths (but not the magnitudes) of these peaks are very similar for all energies and for capture of one or two electrons corresponding also to our measured linear increases in the transverse momentum transfers with increasing projectile velocities. These observations may be ascribed to diffraction limitations which are connected to electron transfer probabilities P(b) which are significant in limited regions of b only. For the H+ + He single-electron capture we observe two additional maxima in the angular differential cross sections. We conclude that while the secondary maxima at ~0.5 mrad probably have large contributions from the Thomas proton--electron--nucleus scattering mechanism, the third maxima at ~0.75 mrad are most likely mainly due to projectile de Broglie wave diffraction.