C A Tachino

Theoretical study of interference effects in single electron ionization of N2 molecules by proton impact

Single ionization of diatomic molecules due to the impact of fast proton beams is theoretically studied. The main interest is focussed on the analysis of the possible existence of interference patterns in the corresponding electron spectra due to coherent emission from the vicinities of the molecular centres. The case of N2 targets is considered. A fully coplanar geometry in which the emitted electron, the incident particle and the molecule are all in the same plane is considered. The dynamics of the reaction is described within the continuum distorted wave-eikonal initial state model, and in the exit channel a two-effective centre approximation is employed. It is shown that fingerprints of coherent electron emission appear in the corresponding angular distributions. Also, double differential cross sections as a function of the energy and of the angle of the ionized electron, obtained by averaging the angular spectra over all molecular orientations, are calculated. Electron emission in opposite phases is found for the two inner 1sσg- and 1sσ*u-orbitals, according with their gerade and ungerade character.

Ionization of water molecules by ion beams. On the relevance of dynamic screening and the influence of the description of the initial state

Single ionization from water molecules by impact of protons, alpha particles and C6 + ions is studied. The post- and prior-versions of the continuum distorted wave-eikonal initial state (CDW-EIS) model within an independent electron approximation are employed to compute double differential cross sections. To avoid the complexity of using numerical molecular continuum states in the cross-section calculations, effective Coulombic continuum wavefunctions are employed. However, this may lead to the appearance of post–prior discrepancies and this fact is examined in detail. Moreover, the influence of the dynamic screening on this behaviour is studied. In addition, the contribution of different molecular orbitals to the angular spectrum is analysed for several ejection electron energies. Finally, the sensitivity of CDW-EIS calculations to the representation of the initial bound molecular orbitals is investigated.

Coherence in collisionally induced electron emission from diatomic heteronuclear molecules

Electron emission from diatomic heteronuclear targets by bare ion impact is analysed. Interference patterns are shown to appear in electron angular distributions, which are explained in terms of coherent emission from the neighbourhood of the atomic centres composing the target. The relationship between electron ionization by ion and circularly polarized photon impact is investigated.

Partial localization in coherent electron emission from asymmetric diatomic molecules

The presence of interference patterns in doubly differential cross sections is investigated for single electron ionization of a heteronuclear system, the HeH+ molecular ion, produced by highly energetic protons. These interferences, which are due to coherent electron emission from the vicinity of the target nuclei, are preserved even after averaging over all molecular orientations. However, when compared with the homonuclear H2 target case, the effect appears to be less pronounced due to the partial localization of the target electrons around the alpha particle centre. Similarities and differences with the case of photon beams are also discussed.

Double differential distribution of electron emission in the ionization of water molecules by fast bare oxygen ions

The doubly differential distributions of low-energy electron emission in the ionization of water molecules under the impact of fast bare oxygen ions with energy of 48 MeV are measured. The measured data are compared with two quantum-mechanical models, i.e. the post and prior versions of the continuum distorted wave–eikonal initial state (CDW-EIS) approximation, and the first-order Born approximation with initial and final wavefunctions verifying correct boundary conditions (CB1). An overall excellent qualitative agreement is found between the data and the CDW-EIS models whereas the CB1 model showed substantial deviation. However, the detailed angular distributions display some discrepancies with both CDW-EIS models. The single differential and total cross-sections exhibit good agreement with the CDW-EIS models. The present detailed data set could also be used as an input for modeling highly charged ion induced radiation damage in living tissues, whose most abundant component is water. Similar measurements are also carried out for a projectile energy of 60 MeV. However, since the double differential cross-section data show similar results the details are not provided here, except for the total ionization cross-sections results.

Investigation of the interference effect in the case of low energy electron emission from O2 in collisions with fast bare C-ions

We have measured the double differential cross sections (DDCSs) for low energy electron emission from O2 under the impact of 51 MeV bare carbon ions. This study is aimed at investigating the Young-type interference in electron emission from a multi-electronic diatomic molecule. The DDCS spectra, differential in emission energy and angle, are compared with state-of-the-art continuum distorted wave-eikonal initial state (CDW-EIS) calculations. The DDCS ratios (i.e. O2/2O) do not produce any oscillatory behaviour due to the interference effect unlike that observed in the case of H2. The forward–backward angular asymmetry parameter, deduced from the measured DDCS values, is found to be a monotonically increasing function of electron velocity and does not show any oscillation. These observations are in qualitative agreement with the prediction of the molecular CDW-EIS model which uses a linear combination of atomic orbitals. The apparent absence of the oscillation in the spectra is qualitatively explained in terms of cancellation of contributions arising from different molecular orbitals.

Single- and two-centre effects in fully differential cross sections for single ionization of H2 molecules by 75keV protons

We present theoretical calculations of single ionization of H2 molecules by 75 keV proton impact. The computed fully differential cross sections for different electron ejection geometries and projectile kinematical conditions are compared with recent measurements made by Hasan et al (2014 J. Phys. B: At. Mol. Opt. Phys. 47 215201). We employ a molecular version of the continuum distorted wave-eikonal initial state model, where all the interactions present in the exit channel are considered on an equal footing. In addition, our approach allows us to incorporate different interference terms and to assess their influence. Overall, the agreement between experiment and theory is better than for the case of more sophisticated schemes for coplanar geometries.

Distorted wave calculations for electron loss process induced by bare ion impact on biological targets

Distorted wave models are employed to investigate the electron loss process induced by bare ions on biological targets. The two main reactions which contribute to this process, namely, the single electron ionization as well as the single electron capture are here studied. In order to further assess the validity of the theoretical descriptions used, the influence of particular mechanisms are studied, like dynamic screening for the case of electron ionization and energy deposition on the target by the impacting projectile for the electron capture one. Results are compared with existing experimental data.

Differential electron emission from multi-electronic targets under highly harged bare ion impact

We report a combined experimental and theoretical study of double differential electron emission in ionization of different multi-electronic atoms under fast highly charged bare ion impact. The main aim is to un-derstand the electron dynamics for these targets which is quite different from one or two electron systems.

Differential electron emission in the ionization of Ne and Xe atoms under fast bare carbon ion impact

Measurement of the energy and angular distributions of the double differential cross section (DDCS) of electron emission from Ne and Xe atoms in collision with 5 MeV u−1 bare carbon ions is reported. This study aimed to investigate the electron emission processes in the case of multi-electronic systems. In general, several clear differences between the electron emission spectra of Ne and Xe are found, which indicate the influence of the increasing number of electrons. For instance, the sharp peak due to the binary nature of collision is almost absent in the case of Xe, unlike Ne, which could be understood due to the increasing contribution from the strongly bound inner shell (such as ) electrons for the Xe atom. The forward–backward angular asymmetry has also been derived from the angular distributions. For Xe, the qualitative behaviour of the asymmetry parameter is seen to be quite different since it reveals structures due to Auger contributions. It is, in general, different and much lower than that for Ne, which shows the smooth behaviour that one finds for other lighter atoms like He. The single differential and total cross sections are also derived. The theoretical calculations based on the prior form of the continuum distorted wave-eikonal initial state (CDW-EIS) approximation have been provided for both the targets. Overall, it gives a very good agreement with the energy and the angular distributions of DDCS for Ne. For Xe, the agreement is not as good as for Ne. We also provide a detailed discussion on the DDCS obtained from different sub-shell ionization, estimated in this framework.