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Collisional excitation of helium atoms by protons of intermediate energies (v p ∼ 1 au) is investigated experimentally. By measuring the intensities of various spectral lines as functions of an electric field applied parallel or antiparallel to the proton beam, we found that the excited He atoms are left after the collision in transient states with large electric dipole moments directed upstream. The experimental results are explained by assuming that during the collision one electron is promoted on the saddle of the two-centre Coulomb potential of the projectile and target. At intermediate energies saddle dynamics is particularly effective due to the stabilization of the electron's motion on the saddle by the Paul-trap mechanism.

The angle, energy, and fluence dependence of electron emission following the interaction of normally incident 100-MeV Ne + ions with thin polypropylene foils and 170-MeV Ne + projectiles with Mylar foils has been investigated experimentally.Spectra were taken for electron ejection angles of 0', 45', and 120' at

Angle-and energy-dependent cross sections for electron emission were measured for 68-MeV/u Kr 33ϩ ions impacting on H 2 . These results show, in accordance with our earlier observation, that interference effects are produced by the coherent emission of electrons from the two H atoms, in analogy with Young's two-slit experiment. Furthermore, the present results demonstrate that the observed oscillatory pattern varies with the electron observation angle, contrary to our earlier expectations but in agreement with recent theoretical predictions.

Interference structures associated with the emission of electrons from H 2 by fast Kr 33ϩ ions (v/cϭ 0.3) are found to exhibit oscillations of second order superimposed on the main oscillatory structure. The secondary oscillations occur with about twice the frequency of the main oscillations. While the primary structure is produced by the coherent emission of electrons from the two atomic centers, similar to Young's two-slit experiment, our theoretical analysis indicates that the frequency doubling is a second-order effect, where the electron wave emitted at one center interferes with the wave backscattered at the other center.