M. A. Abdallah

Capture and ionization processes studied with COLTRIMS

Abstract A review of some basic features of the COLTRIMS technique is presented. Examples of recent work on low-energy electron capture and ionization collisions and for high-velocity ionization from Kansas State University are given.

Low-velocity ionization in asymmetric systems: a comparative study of the He+-Ne and Ne+-He cases

Single target ionization was investigated in the asymmetric collision systems He+-Ne and Ne+-He at projectile velocities of 0.25, 0.35 and 0.5 au employing electron and recoil momentum imaging techniques. At the two lower velocities the recoil transverse momentum distributions were found to be donut shaped. In addition, a concentric double-donut was observed in the He+-Ne system at 0.25 au. At 0.35 and 0.5 au the electron distributions are characterized by a pronounced asymmetry with respect to the beam axis, an asymmetry which is almost completely reversed by exchanging projectile and target. This latter finding is interpreted as confirmation of the molecular nature of the ionization process at these velocities.

Intermediate energy ionization of helium by proton impact

We have investigated impact ionization of He by protons at energies between 20 and 100 keV. Momentum spectra of the ejected electrons were measured for experimentally determined vector impact parameters using cold target recoil ion momentum spectroscopy techniques. At the lowest impact energy, the electron momenta lie close to the saddle point and as the energy increases they slowly move towards the target centre. The measurements are compared with the results of a theoretical calculation carried out using a two-centre momentum space discretization method. Qualitative agreement with the experiment is seen, and systematic disagreements between experiment and theory are discussed.

Experimental evidence of transfer excitation in Ar6+-He collisions

High-resolution cold-target recoil-ion momentum spectroscopy (COLTRIMS) has been used to study single-electron capture processes in collisions of Ar 6+ ions with He at an impact energy of 9 keV and scattering angles between 0 and 3 mrad. The energy-gain spectra show that the capture occurs mainly into the 4s state with a significant contribution involving capture into the 4p state. Reaction channels associated with transfer excitation into the 3s3p3d states of Ar 5+ are also observed for the first time, accounting for 6 .0:4/% of the total cross sections for single- electron capture. Experimental results are found to be in good agreement with the two-electron atomic orbital close-coupling calculations.

Recent recoil ion momentum spectroscopy experiments at KSU

Recoil momentum spectroscopy is used to study collisions involving both fast and slow projectiles on He targets. Experiments have been performed on electron capture and loss from fast ions from the KSU LINAC and slow ions from the KSU CRYEBIS using a supersonic jets with a momentum resolution below 0.5 au. Using fast ions, the final states populated in electron capture from He by 10 MeV F8+ have been resolved with a Q-value resolution of 18 eV, sufficient to separate final channels in which the He+ ion is left excited from those in which He+ is left in its ground state. With slow ions, electron capture from He by slow bare Ne ions has been studied. A few recent results are discussed.Recoil momentum spectroscopy is used to study collisions involving both fast and slow projectiles on He targets. Experiments have been performed on electron capture and loss from fast ions from the KSU LINAC and slow ions from the KSU CRYEBIS using a supersonic jets with a momentum resolution below 0.5 au. Using fast ions, the final states populated in electron capture from He by 10 MeV F8+ have been resolved with a Q-value resolution of 18 eV, sufficient to separate final channels in which the He+ ion is left excited from those in which He+ is left in its ground state. With slow ions, electron capture from He by slow bare Ne ions has been studied. A few recent results are discussed.

Photo- and charged-particle-ionization of He and D2 studied with COLTRIMS

The COLTRIMS (COLd Target Recoil Ion Momentum Spectroscopy) approach to final-state momentum imaging is now being widely used in at least a dozen accelerator and synchrotron-radiation laboratories in the world and its use is growing rapidly. The technique combines fast imaging detectors with a supersonically cooled gas target to allow the charged particles from a collision, including both recoil ions and electrons, to be collected with extremely high efficiency and with fully measured vector momenta. It allows the investigation of correlations between ejected momentum fragments and in some cases the identification of collective modes of disintegration. When molecular targets are used, it allows the a posteriori determination of the alignment of the molecule at the time of the collision. We will discuss the use of this approach to study the single and double ionization of He and D2 by the impact of photons and of charged particles over a wide range of velocities.

Three-dimensional momentum distribution of low energy electrons ejected by fast projectiles

An imaging technique for electron spectroscopy has been developed which measures three dimensional momentum distributions for continuum electrons. Two-dimensions are obtained using position information from a channel plate detector; for the third dimension, electron time of flight measurements are made possible by using the KSU LINAC to time focus projectile ion bunches. Bunch widths of less than 400 psec have been observed. This time of flight combined with position information yields full three-dimensional momenta of ejected electrons. Results for collisions between 30 MeV carbon ions and targets of helium and hydrogen gas are presented.