Ahmad Hasan

Separation of single-and two-center interference in ionization of H2 by proton impact

We present a triple differential experimental study of ionization of molecular hydrogen by proton impact. By comparing cross-sections obtained for coherent and incoherent projectile beams we were able to extract contributions from interference. Two types of distinctly different interferences could be identified. We demonstrate that both types can be separated in the same data set by analyzing triple differential cross-sections for fixed momentum transfer and for fixed recoil-ion momentum.

Triple Differential Study of Ionization of H 2 by Proton Impact for Varying Electron Ejection Geometries

We have performed a kinematically complete experiment on ionization of H2 by 75 keV proton impact. The triple differential cross sections (TDCS) extracted from the measurement were compared to a molecular 3-body distorted wave (M3DW) calculation for three different electron ejection geometries. Overall, the agreement between experiment and theory is better than in the case of a helium target for the same projectile. Nevertheless, significant quantitative discrepancies remain, which probably result from the capture channel, which may be strongly coupled to the ionization channel. Therefore, improved agreement could be expected from a non-perturbative coupled-channel approach.

Fully differential study of wave packet scattering in ionization of helium by proton impact

We present a fully differential study of projectile coherence effects in ionization in p + He collisions. The experimental data are qualitatively reproduced by a non-perturbative ab initio time-dependent model, which treats the projectile coherence properties in terms of a wave packet. A comparison between first- and higher-order treatments shows that the observed interference structures are primarily due to a coherent superposition of different impact parameters leading to the same scattering angle. Higher-order contributions have a significant effect on the interference term.

Influence of the post-collision interaction on interference effects in ionization of H2 by proton impact

We have performed a kinematically complete experiment on ionization of H2 by 75 keV proton impact leading to electrons with a speed equal to the projectile speed. By comparing cross sections measured with a coherent and an incoherent projectile beam we were able to perform a detailed analysis of interference effects. We found that the interference structure is significantly more damped than for a smaller electron energy studied previously. This damping is further increased if kinematic conditions are selected which favor a strong role of the post-collisional interaction between the scattered projectile and the electron ejected to the continuum by a preceding primary interaction with the projectile.

Three-dimensional fully differential single ionization cross sections for 75 keV p + He collisions

We have studied fully differential cross sections (FDCS) for single ionization in 75 keV p + He collisions by measuring the recoil-ion momentum and the projectile momentum in coincidence. The measured FDCS are completely dominated by the binary peak while the recoil peak is absent. This is a clear signature of the post-collision interaction between the outgoing projectile and the ejected electron. Furthermore, our continuum distorted wave calculation demonstrates the importance of the projectile?target nucleus interaction. While the shape of the experimental FDCS is well reproduced by this calculation, there are significant discrepancies in magnitude.

Fully differential study of ionization in p + H2 collisions near electron-projectile velocity matching

We have performed a kinematically complete experiment on ionization of H2 by 75 keV proton impact for electrons ejected with a speed close to the projectile speed. The fully differential data are compared to a three-body distorted wave and a continuum distorted wave—eikonal initial state calculation. Large discrepancies between experiment and theory, as well as between both calculations, are found. These probably arise from a strong coupling between the ionization and capture channels, which is not accounted for by theory.

Projectile Charge Effects in Multiple Ionization of Argon by Positron and Electron Impact

Differential single‐ and multiple‐ionization cross section ratios have been measured for 750 eV positron and electron impact on argon as a function of the projectile energy‐loss from threshold up to 85% of the initial projectile energy. In addition, differential multiple ionization cross section ratios have been obtained for 500 eV electron impact as a function of both the scattering angle and projectile energy loss. The differential single‐ionization cross sections obtained with positrons and electrons are found to have the same shape, i.e., the same energy loss dependence, in agreement with distorted wave Born predictions. The differential double‐ and triple‐ionization cross sections obtained by electron impact, however, are systematically larger than those obtained with positron impact. This charge effect has been attributed to the quantal interference between the mechanisms leading to double ionization.