Electron Capture in Collisions of Slow Highly Charged Ions with an Atom and a Molecule: Processes and Fragmentation Dynamics

International audienceProcesses involved in slow collisions between highly charged ions (HCI) and neutral targets are presented. First, the mechanisms responsible for double electron capture are discussed. We show that, while the electron-nucleus interaction is expected to be dominant at projectile velocities of about 0.5 a.u., the electron-electron interaction plays a decisive role during the collision and gains importance when the projectile velocity decreases. This interaction has also to be invoked in the capture of core electrons by HCI. Finally, the molecular fragmentation of H2 following the impact of HCI is studied

Autoionization electrons following double excitation of D2 in 2.4keV e−+D2 collisions: Experimental and theoretical evidence

International audienceThe electron spectra originating from ionization of D2 by impact of 2.4 keV electrons have been measured at various emission angles. After subtraction of the direct ionization background, three peaks are clearly visible in the electron energy range 1–20eV. The existence of these peaks is confirmed by theoretical calculations. Two of these peaks are due to autoionization of doubly excited states lying above the first and second ionization thresholds. The third peak is attributed to the strong interference between direct ionization and autoionization while the molecule is already dissociating

Anisotropic proton emission after fragmentation of H2O by multiply charged ions

International audienceIon-induced fragmentation of H2O molecules was investigated experimentally by 3He^2+ impact as a function of the energy in the range from 1 to 5 keV, as well as a function of the charge state of 20 keV 20Ne^q+ projectiles (q = 3 and 7). The fragments were detected in the angular range from 25° to 135° with respect to the incident beam direction. Particular emphasis is given to protons originating from collisions at large impact parameters involving a Coulomb explosion mechanism. Absolute cross sections dσ/dΩ, differential in the emission angle, are found to be anisotropic. Protons are preferentially emitted at angles near 90°, with cross sections being ~50% larger than those at forward and backward angles. Possible mechanisms causing anisotropic emission of protons from fragmenting H2O are discussed

Fragmentation of water molecules in slow He2+ + H2O collisions

International audienceThe fragmentation in the collision system 3He2+ + H2O has been investigated experimentally at projectile energies ranging from 1 keV to 5 keV. The fragments were detected at angles from 25° to 130°. The experimental spectra exhibit two groups of peaks. The first one, which extends up to ~30 eV, is interpreted in terms of a Coulomb explosion mechanism. A second group observed at higher energies, corresponds to fragments and scattered projectiles produced in quasi-binary collisions. In the analysis, particular attention is paid to these energetic ions. Absolute cross sections dσ/dΩ, differential in the observation angle, are found to be in good agreement with calculations assuming that the interaction of the screened nuclei is described by the ZBL (Ziegler, Biersack and Littmark) potential

Proton emission following multiple electron capture in slow N7+ + HCl collisions

International audienceCollisions between 98-keV N7+ ions and a HCl target have been investigated experimentally. The kinetic-energy distribution of fragment H+ ions originating from multiple electron capture was detected at angles in the range 20°–160° with respect to the incident beam direction. Proton energies as large as 100 eV were observed, and calculations made in the simple Coulomb explosion model suggest that up to seven target electrons may be involved during the collision. Using the Landau-Zener model, we show that the N7+ projectile mainly captures outer-shell electrons from HCl. From the experimental data we derived multiple-capture cross sections which we compared with results from a model calculation made using the classical over-barrier model and also with a semiempirical scaling law. For the specific case of double capture, several structures appeared, which were assigned using ab initio calculations to states of HCl2+

Electron Capture in Collisions of Slow Highly Charged Ions with an Atom and a Molecule: Processes and Fragmentation Dynamics

Abstract: Processes involved in slow collisions between highly charged ions (HCI) and neutral targets are presented. First, the mechanisms responsible for double electron capture are discussed. We show that, while the electron-nucleus interaction is expected to be dominant at projectile velocities of about 0.5 a.u., the electron-electron interaction plays a decisive role during the collision and gains importance when the projectile velocity decreases. This interaction has also to be invoked in the capture of core electrons by HCI. Finally, the molecular fragmentation of H2 following the impact of HCI is studied

Angular and high-frequency analysis of electron interference structures in ∼60 MeV/u Kr34+ + H2 collisions

International audienceDetailed analyses of previous measurements, combined with additional measurements reported here, have been made of electron interference structures observed in 60 MeV/u Kr34+ + H2 collisions. Results are used to characterize the angular dependence of the primary interference structures over a wide range of electron ejection angles, particularly backward angles, and, additionally, to search for high-frequency structures as reported recently for 1–5 MeV/u H+ + H2 collisions [Hossain et al., Phys. Rev. A 72, 010701(R) (2005)]. The data for backward ejection angles, in combination with earlier data, permit a detailed comparison with theory over the range 30°–150°, showing that none of the existing theories predict accurately the observed oscillation frequencies for backward angles. Electron spectra for 90° and 150° taken in small energy steps and with improved statistics compared to earlier measurements were examined by means of a Fourier analysis but no evidence of the high-frequency structures reported for H+ + H2 collisions was found