Projectile Coherence Effects in Electron Capture by Protons Colliding with H₂ and He

We have measured differential cross sections for single and dissociative capture for 25 and 75 keV protons colliding with H2 and He. Significant differences were found depending on whether the projectile beam was coherent or incoherent. For 75 keV p+H2 these differences can be mostly associated with molecular two-center interference and possibly some contributions from path interference. For 25 keV (both targets) they are mostly due to path interference between different impact parameters leading to the same scattering angles and, for the H2 target, possibly some contributions from molecular two-center interference

Fully Differential Study of Interference Effects in the Ionization of H₂ by Proton Impact

We have measured fully differential cross sections for ionization of H2 by 75-keV proton impact. The coherence length of the projectile beam was varied by changing the distance between a collimating slit and the target. By comparing the cross sections measured for large and small coherence lengths pronounced interference effects could be identified in the data. A surprising result is that the phase angle in the interference term is primarily determined by the momentum transfer and only to a lesser extent by the recoil-ion momentum

Projectile Coherence Effects in Simple Atomic Systems

Recent studies of projectile coherence effects in ion-atom collisions are presented. For intermediate-energy proton collisions an extensive literature provides strong support for the importance of such effects. In this regime coherence effects are now used as a tool to study the few-body dynamics very sensitively. In contrast, for high-energy ion impact the literature is much sparser and here an important role of coherence effects cannot be regarded as being established. In this context, a recent claim that in COLTRIMS experiments the coherence properties are determined only by the target beam is rebutted

Complete Momentum Balance in Ionization of H₂ by 75-keV-Proton Impact for Varying Projectile Coherence

We report on a kinematically complete experiment on ionization of H2 by proton impact. While a significant impact of the projectile coherence properties on the scattering-angle dependence of double-differential cross sections (DDCSs), reported earlier, is confirmed by the present data, only weak coherence effects are found in the electron and recoil-ion momentum dependence of the DDCSs. This suggests that the phase angle in the interference term is determined primarily by the projectile momentum transfer rather than by the recoil-ion momentum. We therefore cannot rule out the possibility that the interference observed in our data is not primarily due to a two-center effect

Measurements of the Effective Electron Density in an Electron Beam Ion Trap using Extreme Ultraviolet Spectra and Optical Imaging

In an electron beam ion trap (EBIT), the ions are not confined to the electron beam, but rather oscillate in and out of the beam. As a result, the ions do not continuously experience the full density of the electron beam. To determine the effective electron density, ne,eff, experienced by the ions, the electron beam size, the nominal electron density ne, and the ion distribution around the beam, i.e., the so-called ion cloud, must be measured. We use imaging techniques in the extreme ultraviolet (EUV) and optical to determine these. The electron beam width is measured using 3d → 3p emission from Fe xii and xiii between 185 and 205 Å. These transitions are fast and the EUV emission occurs only within the electron beam. The measured spatial emission profile and variable electron current yield a nominal electron density range of ne ∼ 1011–1013 cm−3. We determine the size of the ion cloud using optical emission from metastable levels of ions with radiative lifetimes longer than the ion orbital periods. The resulting emission maps out the spatial distribution of the ion cloud. We find a typical electron beam radius of ∼60 μm and an ion cloud radius of ∼300 μm. These yield a spatially averaged effective electron density, ne,eff, experienced by the ions in EBIT spanning ∼ 5 × 109–5 × 1011 cm−3

Corrigendum: Separation of Single- and Two-Center Interference in Ionization of H₂ by Proton Impact (Journal of Physics B: Atomic, Molecular and Optical Physics (2015) 48 (071001))

In a recent Fast Track Communication we presented fully differential cross sections for ionization of H2 by 75 keV proton impact. Among other quantities the data were presented for a fixed energy loss of ε = 57 eV and fixed momentum transfers q. Unfortunately, the values of q which were provided, 0.71, 0.9 and 1.21 a.u., hold for previously published data for = 30 eV, but are incorrect for ε = 57 eV. The correct values are q = 1.25, 1.4, and 1.6 a.u

Triple Differential Study of Ionization of H₂ 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

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