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Experimental cross sections are presented for single-and double-electron capture by C q+ , N q+ , and O q+ ͑q =2-6͒ ions from He at incident energies of ͑1.0-1800͒q eV. Measurements were performed by using a mini-electron-beam-ion-source apparatus in combination with an octopole-ion-beam-guide. The cross sections are found to vary significantly depending on the collision energy as well as on the projectile species. Some cross sections reveal minima at incident energies of a few eV/ u, below which the cross sections increase with decreasing incident energy. To account for the experimental results, we developed a formalism of velocitydependent capture cross sections within the framework of the classical over barrier model by employing an induced dipole interaction potential between the collision partners. In the calculations, doubly excited levels of projectile ions, formed via simultaneous excitation of the outermost projectile electron, were taken into consideration. It is found that the present model can satisfactorily reproduce the experimentally obtained energy dependence as well as absolute values of the cross sections. We conclude that the attractive induced dipole potential and simultaneous projectile excitation are important in low energy electron-capture collisions.

The authors have used a pulsed crossed beam technique incorporating time-of-flight spectroscopy of the collision products to study the electron impact ionization of ground state Fe atoms. Relative cross sections sigma n for the formation of 1 to 4 times ionized iron have been measured within the energy range 8-1250 eV. Individual cross sections have been obtained by normalization to lower energy values of sigma 2 previously measured by Freund et al. (1990) using a fast crossed beam experiment where analysis was complicated by the presence of metastable atoms in Fe beams. Measured cross sections exhibit evidence of contributions from inner shell electrons. The authors' high energy values of sigma 1 are in excellent agreement with theoretical predictions based on the first Born approximation.

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The total attenuation cross section sigma att and the single-electron capture cross section sigma 21 in collisions of 3He2+ with H2 have been measured using a tandem mass spectrometer whose collision cell consists of an octopole ion beam guide. A possible scattering loss which might contribute to sigma att is examined, and a large part of the difference sigma att- sigma 21 is attributed to the double-electron capture process. As its magnitude is as large as 10-15 cm2 at the lowest energy studied. The importance of the process is emphasized in connection with the production of fast protons through Coulomb explosion of H2+2.