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

K-shell and total ionization cross sections following electron-molecule collisions: An empirical scaling law

International audienceCollisions between electrons and various molecular targets (H2O, CH4, C3H4, N2) at projectile energies above the K-shell ionization threshold of the molecule have been investigated experimentally. From electron emission spectra, relative total ionization cross section σt and K-shell ionization cross section σK are determined. The ratio σK/σt is then deduced for each target as a function of the projectile energy and compared with those evaluated in the case of atomic targets. Strong differences between atomic and molecular targets are observed in the slope of the ratio at the highest projectile energies. These differences are explained using the well-known Kim-Rudd formula developed for atomic targets. In the projectile energy range we explored, we develop a simple empirical scaling law for the ratio σK/σt as a function of the projectile energy

lnterférences de type Young avec une source à un seul électron

International audienceDes interférences dues à la diffusion d'un électron unique sur deux protons, qui jouent le rôle de deux trous de Young, ont été mises en évidence expérimentalement pour la première fois. L'électron provient de l'auto-ionisation de l'hélium excité après capture des deux électrons de la molécule H2 par un projectile He2+. Ces interférences se manifestent par l'apparition d'oscillations d'intensité dans la distribution angulaire des électrons diffusés. La présence de ces oscillations prouve qu'un électron peut interférer avec lui-même. L'expérience présente est analogue à une expérience de pensée imaginée par Feynman en 1963

Physique/Physique subatomique. Interférences de type Young avec une source à un seul électron

RESUME : Des interférences dues à la diffusion d'un électron unique sur deux protons, qui jouent le rôle de deux trous d'Young, ont été mises en évidence expérimentalement pour la première fois. L'électron provient de l'autoionisation de l'hélium excité après capture des deux électrons de la molécule H2 par un projectile He2+. Ces interférences se manifestent par l'apparition d'oscillations dans la distribution angulaire des électrons diffusés. La présence de ces oscillations prouve qu'un électron peut interférer avec lui-même. Cette expérience est analogue à l'expérience de pensée imaginée par Feynman en 1963, dans laquelle il retrace le devenir d'un électron après traversée de deux fentes rapprochées. Pour citer cet article : F. Frémont et al., C. R. Physique 9 (2008). ABSTRACT : Young-type interference with a one electron source. In the present work we provide experimental evidence for Young-type interferences caused by one single electron acting on a given double-center scatterer which is analogous to an atomic-size doubleslit apparatus. The interfering electron is provided by autoionization of a doubly-excited helium atom following the capture of the two H2 target electrons by a He2+ incoming projectile ion. In the backward direction, the auto-ionized electron scatters on the two H+ centers of the fully ionized target molecule. Here, the auto-ionizing projectile plays the role of a single-electron source, independent of the interferometer provided by the residual two-center target. The present experiment resembles the famous “thought experiment” imagined by Feynman in 1963, in which the quantum nature of the electron is illustrated from a Younglike double-slit experiment. Similarly to the case of Young's experiment with light, the interference effect manifests itself in well defined oscillations in the angular distribution of the scattered electrons. To cite this article: F. Frémont et al., C. R. Physique 9 (2008)

Coulomb explosion and binary encounter processes in collisions between slow ions and small molecules of biological interest

International audienceIn this work we study the ion impact induced fragmentation of small molecules, which are relevant for radiation damage studies in biological tissues. We present double differential ion emission yields for collisions of N6+ ions with water and methane molecules at 15 and 30 keV impact energies. The angular distribution of the fragment ions shows post-collision and nucleus-nucleus binary collision effects. In the multiple capture energy range, a strong interplay is indicated between the Coulomb explosion and the binary collision mechanisms. In the energy region, where triple capture is dominant, an unexpected angular distribution was found for water fragments, which may be attributed to orientation sensitivity of some of the capture channels. Such processes are relevant for astrophysics and radiation therapy

Anisotropic ion emission in the fragmentation of small molecules by highly charged ion impact

International audienceIn this work, we present double differential ion emission yields measured by the method of fragment ion spectroscopy for collisions of N6+ and O7+ ions with water and methane molecules at ~30 keV impact energies. The angular distribution of the fragment ions shows expected post-collision and nucleus-nucleus binary collision effects with the indication of an interplay between Coulomb explosion and binary collision mechanisms. We have found significant differences in the fragmentation patterns of methane for the two projectiles, which may be a signature of strong, resonant capture channels. For triple capture, an unexpected angular distribution has been found for water, which may be attributed to orientation sensitivity of some of the capture channels. Such processes can be highly important for radiation therapy and astrophysics