Direct Ionization Cross Sections Research Articles

Positron interactions with vinyl acetate from 0.1 eV to 5 keV

Abstract Scattering cross sections from positron impact on vinyl acetate are explored in the energy region 0.1 eV to a 5 keV, employing a cc-pVTZ basis set. The optimized molecular wavefunction of the target was obtained through a multi-center expansion of Gaussian-type orbitals in the Hartree–Fock self-consistent field framework. The elastic cross sections are computed using the single-centre-expansion formalism. Two distinct models were employed to address the long-range effects associated with the target's polar nature and yielded almost identical corrections. The Born-corrected elastic cross sections align more strongly with the existing experimental corrected data than the results reported from the independent-atom-model approximation. The differential and momentum transfer cross sections after applying Born-correction are also reported. The direct ionization cross sections are obtained using the binary-encounter-Bethe model for positrons. The cross sections obtained by summing elastic and ionization cross sections align closely with ‘forward angle corrected’ experimental total cross sections across a significant energy range. The agreement significantly improves beyond 30 eV, suggesting that the omission of excitation and positronium formation cross sections becomes less significant. A brief analysis of the electron interaction with the target is also made.

Calculations of positron scattering from F, F2, HF, and various fluorocarbons

Abstract Single center convergent close-coupling (CCC) calculations have been completed for positron scattering from atomic fluorine. Total, electron-loss, positronium-formation, direct ionization, momentum transfer, elastic, bound-state excitation, and stopping power cross sections have been determined for energies between threshold and 5000 eV. Past calculations for this scattering system exist only for elastic and momentum-transfer cross sections. For high energies, good agreement is found between current and past results. At low energies, however, large differences are found between the current calculations and previous results. The atomic fluorine results are then used in a modified independent atom approach to calculate cross sections for positron scattering on F2, HF, CF4, C2F6, C3F6, C3F8, and C6F6. The current molecular results are typically higher than previous positron experiments across the calculated energy range, however, these experiments were not corrected for the forward angle scattering effect and likely underestimate the true result. Good agreement is found between the current positron results and previous electron experiments and calculations at high energies.

Positron scattering from neon and argon

The relativistic optical potential method has been used to calculate a complete set of cross sections for positron scattering from neon and argon in the energy range from zero to 1 keV. Elastic, positronium formation, excitation, direct ionization, and grand total cross sections are presented and compared to the recommended results of Ratnavelu et al. (J Phys Chem Ref Data 48:023102, 2019). Here, positronium formation is modeled as an ionization process by successively lowering the ionization thresholds. This process was carried out for the four lowest energy levels of positronium.Graphical abstract

Convergent close-coupling calculations of positron scattering from neon and argon

Positron scattering from the noble gas atoms neon and argon is calculated using the single-center convergent close-coupling (CCC) method. Unlike previous CCC calculations, we have relaxed the frozen-core approximation by fully opening the outermost s and p shells, and find better agreement with experiment for excitation energies and dipole polarizability. We have used this improved structure model to calculate elastic, elastic differential, total, momentum-transfer, electron-loss, and total bound excitation cross sections. We have applied a complex model potential calculation, scaled to our single-center CCC results, to obtain direct ionization and positronium-formation cross sections for these systems. This complex model potential was also used to calculate results between the positronium-formation and ionization thresholds, giving complete results from 10-5\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$10^{-5}$$\\end{document} eV to 5000 eV. Overall, good agreement is observed with past theory and experiment for most transitions.Graphic abstract

Positron scattering from krypton and xenon

A cross section set is reported for positron scattering from krypton and xenon over the energy range from zero to 1 keV using the relativistic optical potential method. This set includes elastic, positronium formation, excitation, direct ionization, and grand total cross sections. They are compared to the recommended results of Ratnavelu et al. (J Phys Chem Ref Data 48:023102, 2019). The positronium formation cross section reported is represented by the sum obtained by successively lowering the ionization threshold to coincide with those for formation into the four lowest energy levels of positronium.Graphical

Electron- and positron-driven molecular processes for H2O, CO2, and NH3 in their gas and ice phases.

In this comprehensive study, we explored the molecular chemistry of H2O, CO2, and NH3 molecules in their gas and ice phases assisted by electron and positron interactions over a wide range of energy from threshold to 5000 eV. These simple molecules have immense applications in areas such as atmospheric sciences and astrochemistry. We employed the spherical complex optical potential (SCOP) model along with the complex scattering potential-ionization contribution (CSP-ic) model and modified them to quantify the probabilities of various charged particle-driven molecular processes through inelastic cross-sections (Qinel), total ionization cross-sections (Qion), direct ionization cross-sections (QD-ion), and positronium formation cross-sections (QPs) for these molecules in gas and ice phases. We propose a novel model that incorporates the band gap of a material for positron impact cross-sections with molecules in their condensed phase. This is the first study on electron interactions with NH3 (ice) and positron interactions with H2O, CO2, and NH3 in their condensed phase.

Theoretical investigation of electron-impact ionization of W<sup>6+</sup> ion

<sec>Due to its unique characteristics, metal tungsten has been selected as the wall material for the tokamak magnetic confinement fusion device. The wall material directly interacts with the plasma for a long time, thus causing tungsten atoms and ions to be sputtered and ionized into different charge states, which then enter the tokamak device as plasma impurities. To ensure stable plasma combustion conditions, highly complex model is currently being used to evaluate the behavior of tungsten impurities and their influence on the tokamak plasma. This requires various high-precision atomic data for tungsten atoms and different ionized states of tungsten ions. Electron collision ionization, as a fundamental atomic physical process, is widely encountered in laboratory and astrophysical plasma environments. The parameters such as electron collision ionization cross-sections and rate coefficients are crucial for plasma radiation transport simulations and state diagnostics.</sec><sec>Electron-impact single-ionization (EISI) cross sections of the ground state and metastable state for W<sup>6+</sup> ions are calculated by using the level-to-level distorted-wave (LLDW) method. The contributions of direct ionization (DI) cross section and excited autoionization (EA) cross section to the total EISI cross section are primarily considered.</sec><sec>Comparison of our calculation results with the experimental data from Stenke et al. (Stenke M, Aichele K, Harthiramani D, Hofmann G, Steidl M, Volpel R, Salzborn E <ext-link ext-link-type="uri" xmlns:xlink="http://www.w3.org/1999/xlink" xlink:href="https://iopscience.iop.org/article/10.1088/0953-4075/28/13/021">1995 <i>J. Phys. B: At. Mol. Opt. Phys.</i> <b>28</b> 2711</ext-link>) reveals that the EISI cross section considering only the ground state is significantly smaller than the experimental result. Therefore, it is imperative to take into account the contribution from the metastable state. To determine the fraction of ions in long-lived energy levels within the parent ion beam, three models are employed.</sec><sec>Our results, which include the contribution of metastable states, accord well with the experimental results of Stenke et al. Compared with the theoretical calculation result of Pindzola et al. our calculaiton provides a more comprehensive understanding of the electron-impact single-ionization process for W<sup>6+</sup> ions. The comparison is illustrated in the attached figure.</sec>

Theoretical investigations of positron collisions with phosphorus-containing compounds

A theoretical investigation of positron scattering from phosphorus-containing compounds (viz., PH3, P2H4, PCl3, PF3, PBr3, POF3, POCl3, and H2PO4) is reported in this article. The quantum mechanical potential scattering approach is utilized to calculate integral elastic, excitation, momentum transfer, direct ionization, positronium formation, total ionization, inelastic, differential, and total cross sections on a fine energy grid from 1 to 5000 eV. The ionization contribution in the inelastic scattering is estimated using the complex scattering potential-ionization contribution technique. Prior research on positron collisions with these targets is scarce; as a result, the purpose of this study is to make up, at least in part, for this deficiency in cross-section data. In addition to being pertinent to positron transport analyses, such as Monte Carlo methods, the current results should be useful to benchmark the accuracy and validity of positron molecule collision computations and, more significantly, to compare these calculations with related electron scattering outcomes. Furthermore, the calculated cross sections of PH3 are compared with NH3 and other phosphorus-containing compounds. The analysis makes it abundantly evident that the atoms on the periphery of a molecule have a substantially larger impact on the scattering process than the central atom. To analyze the scattering dynamics of positrons and their anti-particle electrons, a comparative study of cross sections of H2PO4 and H2SO4 is also presented. For most of these targets, positron calculations are carried out for the first time.

Convergent close-coupling calculations of positron scattering from atomic carbon

We report on the extension of the single-center convergent close-coupling method to positron scattering on multielectron atoms, along with the development of a technique for separating the direct ionization and positronium-formation channels in single-center calculations. We have performed calculations of positron scattering on carbon and present total elastic, momentum transfer, excitation, direct ionization, total ionization, total inelastic, positronium-formation, stopping power, and total cross sections from threshold to 5000 eV. We also present oscillator strengths, the scattering length, the hidden Ramsauer-Townsend minimum, the energy of the positron-carbon virtual state, and the mean excitation energy. Agreement with electron-scattering experiment and positron-scattering theory for several cross sections has been demonstrated for high energies. However, discrepancies exist between different theoretical methods at lower energies and for the ionization and positronium-formation processes.

Electron-impact ionization of the tantalum atom

Electron-impact ionization cross sections for the ground configuration of the tantalum (Ta) atom are calculated using a combination of non-perturbative time-dependent close-coupling and perturbative distorted-wave methods. Direct ionization of the 6s and 5d subshells leading to single ionization are presented. The inclusion of a polarization potential in the 6s direct ionization leads to a small reduction in the cross section. Angular factors are included that allow the configuration-average direct ionization cross sections to be split into LS term resolution. The magnitude of the ionization cross section suggests that the ionization mean free path for neutral Ta will be similar to that for tungsten (W) plasma facing components in fusion experiments, leading to similarly large fractions of prompt redeposition.

Binary-Encounter Model for Direct Ionization of Molecules by Positron-Impact

We introduce two models for the computation of direct ionization cross sections by positron impact over a wide range of collision energies. The models are based on the binary-encounter-Bethe model and take into account an extension of the Wannier theory. The cross sections computed with these models show good agreement with experimental data. The extensions improve the agreement between theory and experiment for collision energies between the first ionization threshold and the peak of the cross section. The models are based on a small set of parameters, which can be computed with standard quantum chemistry program packages.

Significance of the high charge state of projectile ions inside the target and its role in electron capture leading to target-ionization phenomena

The $K$ x-ray spectra of different targets (Cu, Zn, and Ge) induced by 3--5 MeV/u Si projectile ions have been measured to determine the $K$-shell ionization cross section. A significant difference is observed between the measurements and theoretical estimates, with the latter being about 50% below the experimental results. This underestimation is attributed to the charge exchange from the target $K$ shell to projectile $K$ and $L$ shells. Such an observation can only be possible if the projectile ions attain up to H- and He-like charge states. Corresponding projectile charge state fractions have been evaluated from the Lorentzian charge state distribution, where the mean charge state is taken from the Fermi gas model [W. Brandt et al., Phys. Rev. Lett. 30, 358 (1973)] and the width from the Novikov and Teplova approach [Phys. Lett. A 378, 1286 (2014)]. The sum of the direct ionization cross section and $K\text{\ensuremath{-}}K+K\text{\ensuremath{-}}L$ capture cross sections gives a good agreement with the measured cross sections. Furthermore, we have validated this methodology with available data for a Si ion on Ti target. Such results may be useful in many solid target-based applications.

Electron-impact ionization for the levels of Fe3＋

Electron-impact single ionization is investigated for the levels of the ground 3d5 and long-lived 3d44s configurations of the Fe3＋ ion. In addition, the Maxwellian rate coefficients are tabulated in the study. The difference among the cross sections reaches ∼11% and ∼17% for the levels of the ground and long-lived configurations, respectively. The excitation autoionization (EA) provides the larger variation of the cross sections compared to the direct ionization (DI). It is demonstrated that the EA cross sections for the levels of the ground configuration vary more strongly compared to the values calculated for the excited configuration. On the other hand, the DI cross sections evaluated for the levels of the ground configuration agree within ∼4% while the difference of ∼9% is obtained for the levels of the long-lived configuration.

Electron-Impact Ionization of Heavy Atoms Using the Time-Dependent Close-Coupling Method

The time-dependent close-coupling method has been recently applied to calculate electron-impact direct ionization cross sections for the Kr, W, and Pb atoms. An overview is presented for these three heavy neutral atom systems. When the direct ionization cross sections are combined with excitation-autoionization cross sections, the total ionization cross sections were found to be in reasonable agreement with crossed-beams measurements for Kr and Pb.

Electron-impact ionization of the Pb atom

Electron-impact ionization cross sections are calculated for the ground configuration of the Pb atom. Time-dependent close-coupling cross sections for the direct ionization of the 6s and 6p subshells leading to single ionization are calculated with and without a polarization potential. Configuration-average distorted-wave cross sections for the direct ionization of the 6s and 6p subshells leading to single ionization are also calculated with and without a polarization potential. We find the time-dependent close-coupling cross sections using a polarization potential to be in good agreement with convergent-close-coupling cross sections using a polarization potential. The total direct ionization cross sections are compared to two sets of experimental measurements. The differences between the direct ionization cross sections and the experimental measurements are mainly due to indirect ionization cross sections coming from the 6s^2 6p^2 rightarrow 6s 6p^3 excitation followed by autoionization.Graphic

Calculation of electron induced ionization cross sections of fusion plasma relevant material: W atoms

The electron induced processes on tungsten (W) and related materials are of prime importance for the fusion plasma application purposes. W and tungsten based materials have been recommended as the plasma facing components such as walls and divertors in the current fusion devices. We report electron impact total ionization cross sections (TICS) for the W atoms. We also report the direct ionization cross sections for the 4f, 5d, 5p and 6s sub shells of the ground state W. The ionization cross sections have been calculated in the variants of distorted wave approximation (DWA) using Hartree-Fock wavefunctions and distorted potential. We have also calculated ionization cross sections in simpler modified Kim and modified Khare models with scaling laws. Present TICS results have been compared with the other available theoretical results. Reasonable agreement with the existing theoretical results have been obtained for the TICS of W atoms by present distorted wave and BEB approaches. The majority of contribution to the total ionization cross section comes from the direct ionization of 6s and 5d subshells.

Energy Dependency of Proton-Induced Outer-Shell Multiple Ionization for 48Cd and 49In

L subshell X-rays of 48Cd and 49In have been measured for the impact of protons with energies from 75 to 250 keV. Obviously, it is found that Lγ2 (abbreviation Lγ2,3 for 48Cd and Lγ2,3,4 for 49In) X-ray emission is enhanced in comparison with Lγ1 X-ray emission. The relative intensity ratios of Lγ2 to Lγ1 X-ray are larger than the atomic data and increase with decreasing proton energy. This is caused by the multiple ionization of outer-shell electrons. To verify this explanation, the enhancements for relative intensity ratio of Lι and Lβ2 to Lα X-ray in experiments are discussed, and the direct ionization cross sections of 4d, 5s, and 5p electrons are calculated using BEA theory.

Direct And Dissociative Ionization Cross Section Of Oxygen Molecule From Threshold To 10 Kev

Due to abundant applications of ionization in various fields of applied sciences, it’s desirable to calculate absolute ionization cross sections of various atoms or molecules. In this literature, we have calculated the absolute direct and dissociative ionization cross sections of the oxygen molecule from threshold energy to 10,000 eV by using revisited Jain-Khare semi-empirical approach. In this literature, the total direct and absolute ionization cross section data have compared with easily available experimental and/or theoretical data. The present results give a better account for the ionization cross sections up to higher incident electron energies.

Electron-impact single ionization ofFe3+from the ground and metastable states

Single ionization by electron impact of the ${\mathrm{Fe}}^{3+}$ ion is investigated using the Dirac-Fock-Slater approximation for the levels of the ground and first-excited $3{d}^{4}4s$ configuration. Contributions from direct ionization (DI) and excitation-autoionization (EA) processes are taken into account. The scaled distorted-wave (DW) cross sections for DI and excitation by electron impact are used to analyze the experimental data. The study demonstrates the importance of correlation effects for the EA process. Good agreement with experimental data reveals that currently available DW cross sections and, therefore, the corresponding Maxwellian rate coefficients for electron-impact ionization process in ${\mathrm{Fe}}^{3+}$ are overestimated and their usage may lead to wrong predictions for charge state distribution in plasma.

Positron scattering from helium

We have used our relativistic optical potential method to calculate a complete set of cross sections for positron scattering from helium over the range from 0 to 1 keV. We present elastic, positronium formation, direct ionization, total ionization and grand total cross sections and compare these to measurements and other calculations. Besides the previous method for calculating positronium formation by lowering the threshold for ionization to coincide with that for positronium formation in its ground state, we have also presented results where this process is represented by including the sum obtained by successively lowering the ionization threshold to coincide with those for positronium formation in the first four excited n levels. Agreement with previous work is generally good.