J. Hanssen

Quantum-mechanical predictions of DNA and RNA ionization by energetic proton beams

Among the numerous constituents of eukaryotic cells, the DNA macromolecule is considered as the most important critical target for radiation-induced damages. However, up to now ion-induced collisions on DNA components remain scarcely approached and theoretical support is still lacking for describing the main ionizing processes. In this context, we here report a theoretical description of the proton-induced ionization of the DNA and RNA bases as well as the sugar-phosphate backbone. Two different quantum-mechanical models are proposed: the first one based on a continuum distorted wave-eikonal initial state treatment and the second perturbative one developed within the first Born approximation with correct boundary conditions (CB1). Besides, the molecular structure information of the biological targets studied here was determined by ab initio calculations with the Gaussian 09 software at the restricted Hartree-Fock level of theory with geometry optimization. Doubly, singly differential and total ionization cross sections also provided by the two models were compared for a large range of incident and ejection energies and a very good agreement was observed for all the configurations investigated. Finally, in comparison with the rare experiment, we have noted a large underestimation of the total ionization cross sections of uracil impacted by 80 keV protons,whereas a very good agreement was shown with the recently reported ionization cross sections for protons on adenine, at both the differential and the total scale.

Electron impact ionization of water molecule

In the present paper, differential and total cross sections are calculated for the interaction of electrons with a water molecule. The calculations are performed in the distorted wave Born approximation framework where the incident and scattered (fast) electrons are described by a plane wave function, whereas the ejected (slow) electron is described by a distorted wave function. From the fivefold differential cross sections, triply and singly differential cross sections have been calculated by successive integrations. In these conditions, very good agreement is found with available experimental measurements essentially limited to triply and singly ionization cross sections. Finally, a comparison of our results with a large set of experimental data of total ionization cross sections exhibits very good agreement.

Interference effects in single ionization of molecular hydrogen by electron impact

A recently developed molecular three-continuum approximation is employed to compute differential cross sections for the ionization of hydrogen molecules by electron impact. Within the framework of this approximation, the chosen final electronic wavefunction takes into account the molecular character of the target as well as the correlate motion between the aggregates in the final channel of the reaction. Fivefold-differential cross sections as a function of both the electron momenta in the final state and the molecular orientation are studied for different kinematical arrangements. Interference structures coming from the two-centre geometry of the molecule are predicted in this case. Integrated cross sections over all molecular orientations are also calculated. It is shown that interference patterns remain, even for this case.

Differential and total (e,2e) cross sections of simple polyatomic molecules

In this paper, we present a theoretical approach to calculate differential and total ionization cross sections of polyatomic molecules by fast electron impact. More exactly, we have studied the ionization of ammonia (NH(3)) and methane (CH(4)) molecules, and previous results concerning the H(2)O molecule ionization are reported for comparison. The calculations are performed in the distorted wave Born approximation without exchange by employing the independent electron model. The molecular target wave functions are described by linear combinations of atomic orbitals. To describe the interaction between the inactive target electrons and the slow ejected electron, we have introduced a distortion via an effective potential calculated for each molecular orbital. The present theoretical calculations agree well with a large set of existing experimental data in terms of multiple differential and total cross sections.

Influence of the dynamic screening on single-electron ionization of multi-electron atoms

A complete formulation of the post-version of the continuum distorted wave-eikonal initial state model to investigate single-electron ionization of multi-electron atoms by fast bare ion beams is considered. The influence of the non-ionized electrons on the dynamic evolution of the ejected electron is analysed showing that the corresponding interaction plays a main role in the determination of double differential cross sections. It is demonstrated that its inclusion as an additional term in the perturbative potential of the exit channel avoids discrepancies between the pre- and post-versions of the studied distorted wave model.

Water versus DNA: new insights into proton track-structure modelling in radiobiology and radiotherapy.

Water is a common surrogate of DNA for modelling the charged particle-induced ionizing processes in living tissue exposed to radiations. The present study aims at scrutinizing the validity of this approximation and then revealing new insights into proton-induced energy transfers by a comparative analysis between water and realistic biological medium. In this context, a self-consistent quantum mechanical modelling of the ionization and electron capture processes is reported within the continuum distorted wave-eikonal initial state framework for both isolated water molecules and DNA components impacted by proton beams. Their respective probability of occurrence-expressed in terms of total cross sections-as well as their energetic signature (potential and kinetic) are assessed in order to clearly emphasize the differences existing between realistic building blocks of living matter and the controverted water-medium surrogate. Consequences in radiobiology and radiotherapy will be discussed in particular in view of treatment planning refinement aiming at better radiotherapy strategies.

The first Born approximation for charge transfer collisions

The authors evaluate the first-order Born perturbation theory for electron capture from atoms by swift ions, with proper allowance for the asymptotic conditions for this three-body Coulomb problem as derived by Belkic et al. (1979). The authors show that the results differ strongly from those of the conventional Oppenheimer-Brinkman-Kramers approximation. Hence the magnitude of the higher terms in the Born series have to be re-evaluated. Some information on this topic is obtained by a comparison between the authors results and those of higher order approximations.

Positronium formation in collisions of fast positrons impacting on vapour water molecules

Positronium formation through electron capture by fast positrons impinging on vapour water molecules is studied theoretically at intermediate and high impact energies. This multi-electron system is treated within the framework of the independent electron model. The charge transfer process is described by employing the continuum distorted-wave final-state approximation. In this model, the final state of the collision is distorted by two Coulomb wavefunctions associated with the interactions of both the positron and the active electron (the captured one) with the residual ionic target. The water molecule is described by an expansion of monocentric Slater-like functions centred on the oxygen atom. Total cross sections are computed by using a partial-wave technique and compared with results obtained using the simpler Coulomb– Born approximation. Theoretical results for the production of H2O + ions in the final channel of the reaction through charge transfer processes (positronium formation and ionization) are also presented.

Fragmentation in collisions of Na9+ clusters with Cs atoms

We have evaluated charge transfer, excitation and fragmentation cross sections in Na9+ + Cs collisions using a molecular close-coupling formalism and a postcollisional rate-equation model. The calculated charge transfer cross sections are in good agreement with recent experimental measurements below v = 0.04 au. We show that the relative abundance of the different fragments depend critically on the cluster temperature and the spectrometer time-of-flight window.

Double electron capture by fast bare ions in helium atoms: production of singly and doubly excited states

Double electron capture by fast bare ions in helium is studied in the framework of the Four-Body Continuum Distorted Wave theory. It is shown that experimental data is well reproduced when the theoretical total cross-section include both contributions from the ground state and from the first singly excited states. In addition, cross-sections for the production of the first doubly excited states are also calculated. Present results call for additional experimental data at higher impact energies than presently available.