The energy distribution of binary-encounter electrons (BEE) produced in collisions of 1–2 MeV/amu ${\mathrm{H}}^{+}$ and bare C, N, O, and F ions with ${\mathrm{H}}_2$ and He gas targets is reported at 0° with respect to the beam direction. These electrons result from ionization of the target due to hard collisions with the projectile and can thus be considered to be produced in a process analogous to elastic scattering of a free electron from a highly charged ion. An impulse-approximation (IA) model has been developed to describe this process in which ‘‘quasifree’’ target electrons undergo 180° Rutherford scattering in the projectile frame. The measured BEE double-differential production cross sections for bare ions were well described by this model and were found to scale with ${\mathit{Z}}_{\mathit{p}}^2$ and ${\mathit{E}}_{\mathit{p}}^{\mathrm{-}(\mathrm{\sim }2.6–2.7)}$ where ${\mathit{Z}}_{\mathit{p}}$ and ${\mathit{E}}_{\mathit{p}}$ are the charge and energy of the projectile, respectively. An energy shift of the BEE below 4t, where t is the cusp electron energy, is observed and is also predicted by the IA treatment. A plane-wave Born approximation (PWBA) calculation for BEE production is also found to be in overall agreement with our data. However, the energy shift of the BEE peak could not be fully accounted for within this PWBA calculation.

Few-photon multiple ionization of Ne and Ar atoms by strong vacuum ultraviolet laser pulses from the free-electron laser at Hamburg was investigated differentially with the Heidelberg reaction microscope. The light-intensity dependence of ${\mathrm{Ne}}^{2+}$ production reveals the dominance of nonsequential two-photon double ionization at intensities of $I<6\times {10}^{12}\mathrm{W}/{\mathrm{cm}}^2$ and significant contributions of three-photon ionization as $I$ increases. ${\mathrm{Ne}}^{2+}$ recoil-ion-momentum distributions suggest that two electrons absorbing “instantaneously” two photons are ejected most likely into opposite hemispheres with similar energies.

Evidence is presented of an excitation process in ion-atom collisions analogous to electron-impact excitation in free-electron-ion collisions. The production of (1s2s2p${)}^4$P projectile states excited in collisions of (1${\mathrm{s}}^2$2s) ${\mathrm{O}}^{5+}$ and ${\mathrm{F}}^{6+}$ with He and ${\mathrm{H}}_2$ targets was found to increase with projectile energy above ∼0.75 MeV/u, in agreement with an impulse-approximation treatment of 1s→2p electron-impact excitation of ions in collisions with ‘‘quasifree’’ target electrons. Such ion-atom excitations could provide presently unavailable information about inner-shell electron-impact excitation.

Double ionization of He by 3-keV electron impact is explored in a kinematically complete experiment. The 12-dimensional, 4-particle final-state momentum space is mapped for two slowly ejected electrons $({\mathbf{k}}_{b,c}<1.2\mathrm{}\mathrm{a}.\mathrm{u}.\mathrm{})$, the ${\mathrm{He}}^{2+}$ ion $({\mathbf{k}}_{\mathrm{He}}<5\mathrm{}\mathrm{a}.\mathrm{u}.\mathrm{})$ and the fast electron for all relative emission angles and energy partitions. Different reaction mechanisms are identified by examining the angular correlation between the slow electrons as a function of the momentum transfer. A fivefold differential cross section is presented for asymmetric energy sharing in coplanar geometry.