Direct Ionization Cross Sections Research Articles

Evaluation of Electron-Impact Ionization Cross Sections for Molecules.

We describe the recent progress in the development of the semiempirical approach developed by Jain and Khare for the calculations of ionization cross sections for molecules by electron impact. Along with the state-of-the-art description of this approach, the emphasis will be on the evaluation of cross sections for a carbon dimer, C2, and trimer, C3. Single-differential ionization cross sections as a function of secondary or ejected electron energy and averaged secondary electron energy for C2 and C3 are calculated at fixed impinging electron energies of 100 and 200 eV. The integral ionization cross sections are also derived from ionization threshold to 2000 eV. Extensive comparisons of the presently evaluated direct ionization cross sections with the only available theoretical binary-encounter-dipole model calculations of Kim and Rudd, spherical-complex-optical-potential model calculations of Nagama and Antony, DM model calculations of Deutsch and Märk, and the calculations of Michelin et al. based on the combination of the Schwinger variational iterative method and distorted wave approximation revealed a satisfactory agreement. The present study also establishes the validity of the semiempirical approaches and so provides a solid foundation for further applications to the larger molecules.

Electron-impact ionization of the Si atom

Electron-impact ionization cross sections are calculated for the ground configuration of the Si atom for application in erosion diagnostics of plasma facing components in magnetically confined fusion experiments. The single ionization cross sections include contributions from outer subshell direct ionization and excitation-autoionization. The dominant direct ionization cross sections are calculated using a non-perturbative time-dependent close-coupling method. A significant reduction in the direct ionization cross section is found when compared to perturbative distorted-wave results. The theoretical cross sections are compared with available crossed-beams measurements.

Theoretical study of positron scattering by group 14 tetra hydrides: A quantum mechanical approach

The present work provides a comprehensive set of positron impact scattering cross sections for group 14 tetrahydrides, namely, SiH4, GeH4, SnH4, and PbH4. The well-established spherical complex optical potential and complex scattering potential-ionization contribution methods are modified to incorporate positron scattering in the present work to calculate various cross sections. The positronium formation channel is adequately included through an improved inelastic threshold. The energy range chosen for the direct ionization cross section is from the respective ionization potential (I) of the molecule to 5 keV. Likewise, positronium formation and total ionization cross sections are reported from the positronium formation threshold to 300 eV and 5 keV, respectively, and the total cross section is computed for an extensive energy range of 1 eV to 5 keV. The positron impact total cross section for stannane molecule is computed for the first time. A characteristic valley is observed in the total cross sections with minima close to the positronium formation threshold. Further increase of cross section signifies the opening of inelastic channels especially positronium formation. In general, a reasonable agreement is found between the present results and other comparisons, wherever available. Furthermore, this is the first report of the inelastic cross sections (direct ionization, positronium formation, and total ionization) for the present set of targets.

Maxwellian rate coefficients for electron-impact ionization of W26+

Excitation-autoionization (EA) and direct ionization (DI) Maxwellian rate coefficients (MRC) are presented for the levels of the ground configuration of the W26+ ion. The study of EA and DI cross sections is performed by applying the Dirac–Fock–Slater method within the level-to-level distorted-wave approximation including radiative damping. Excitations to the high-nl shells (n ≤ 25) are taken into account for the EA process. It is shown that electron-impact ionization from the different levels of the ground configuration produces a significant variation of MRC. Excitations to configurations with energy levels that straddle the ionization threshold play the main role in enhancing MRC for the highest level of the ground configuration. The difference of 40% among the EA MRC is determined for the levels of the ground configuration.

Positron induced scattering cross sections for hydrocarbons relevant to plasma

This article explores positron scattering cross sections by simple hydrocarbons such as ethane, ethene, ethyne, propane, and propyne. Chemical erosion processes occurring on the surface due to plasma–wall interactions are an abundant source of hydrocarbon molecules which contaminate the hydrogenic plasma. These hydrocarbons play an important role in the edge plasma region of Tokamak and ITER. In addition to this, they are also one of the major components in the planetary atmospheres and astrophysical mediums. The present work focuses on calculation of different positron impact interactions with simple hydrocarbons in terms of the total cross section (Qtot), elastic cross section (Qel), direct ionization cross section (Qion), positronium formation cross section (Qps), and total ionization cross section (Qtion). Knowing that the positron-plasma study is one of the trending fields, the calculated data have diverse plasma and astrophysical modeling applications. A comprehensive study of Qtot has been provided where the inelastic cross sections have been reported for the first time. Comparisons are made with those available from the literature, and a good agreement is obtained with the measurements.

Study of elastic and inelastic cross sections by positron impact on inert gases

In this article, a modified computational method recently introduced is used for the calculation of total, positronium (Ps) formation and ionization cross sections including direct and total ionization cross sections for positron scattering from noble gases. The incident positron is assumed to have energies over a wide range from ~5 eV to 5 keV. The positron–atom interaction potential is developed under an optical potential framework and the computations of cross sections for each process are performed by introducing appropriate absorption thresholds. The calculated results obtained by employing this modified approach are found to be in reasonably good agreement with most of the existing data.

Accurate solution of the proton–hydrogen three-body scattering problem

An accurate solution to the fundamental three-body problem of proton–hydrogen scattering including direct scattering and ionization, electron capture and electron capture into the continuum (ECC) is presented. The problem has been addressed using a quantum-mechanical two-center convergent close-coupling approach. At each energy the internal consistency of the solution is demonstrated with the help of single-center calculations, with both approaches converging independently to the same electron-loss cross section. This is the sum of the electron capture, ECC and direct ionization cross sections, which are only obtainable separately in the solution of the problem using the two-center expansion. Agreement with experiment for the electron-capture cross section is excellent. However, for the ionization cross sections some discrepancy exists. Given the demonstrated internal consistency we remain confident in the provided theoretical solution.

Second Born fully differential cross sections for direct ionization of H(1s) by proton and antiproton collisions

Fully differential cross sections for ionization of H(1s) atoms by collision with protons and antiprotons are calculated within the second-order Born closure approximation. The calculations are performed for emission of electrons with different energies into planes, moving from the scattering to the perpendicular planes at different impact energies and small scattering angles from which a significant contribution to the total cross section comes. The influence of the projectile’s charge and the second Born term on the calculations varies according to the incident energy, emission energy, and the scattering angle.

Two-center close-coupling calculations of positron–molecular-hydrogen scattering

A two-center close-coupling method that includes positronium (Ps) formation channels has been developed for positron scattering on molecular hydrogen. Calculations are performed within the fixed-nuclei approximation. Results for the grand total, Ps-formation, and direct ionization cross sections are presented. In general, good agreement with experiment is seen in all the calculated cross sections.

Excitation and ionization of outer shells in Rb by electron impact

The relativistic distorted-wave and binary-encounter-dipole approximations were employed for calculating the electron-impact single ionization cross sections of the 5s, 4p6, 4s2, 3d10 shells and 4p6 excitation cross section for Rb atom taking into account both configuration interaction and relativistic effects. The capabilities of the most used theoretical approaches in describing the single ionization of Rb atom were considered by comparing the present and other available calculated data with the experimental total ionization and total direct single ionization cross sections over the electron-impact energy range from the 5s threshold to 600 eV. The best agreement within experimental uncertainty was obtained by using the non-relativistic binary-encounter-dipole approximation included in the LANL Atomic Physics Codes package. At present none of the used approximations (plane-wave Born or relativistic distorted wave) can satisfactorily describe the experimental excitation-autoionization cross section in rubidium because the efficient formation of the 4p6 core-excited negative-ion rubidium states at near-threshold impact energies is ignored in calculations.

Modified Binary-Encounter-Bethe Model for Electron Impact Ionization Cross Sections of Highly Charged Ions

SynopsisThe modified binary-encounter-Bethe model has been used in the calculation of direct ionization cross sections of ions with several charge states [1], going all the way to hydrogen-like Uranium [2]. In this work we present electron impact cross section values for hydrogen-like Mo, Dy, Au and Bi.

Electron-Impact Ionization of Se16+ and Se17+ Ions

SynopsisElectron-impact ionization cross sections for Se16+ and Se17+ ions have been calculated using semirelativistic configuration-average distorted-wave (CADW) method for both the direct and indirect ionization contributions. The excitation-autoionization contributions originating from the excitations of 2s and 2p subshels were calculated using a semirelativistic level to level distorted-wave (LLDW) method. Direct ionization cross section calculations involving 2s,2p,3s, and 3p subshells were also calculated using the LANL collisional atomic code and comparisons are made.

Multiple ionization of Ne induced by 10 keV/u–500 keV/u Cq+ ( q = 1–3) ions**Project supported by the National Natural Science Foundation of China (Grant Nos. 11174116 and 11175075).

The values of cross-section ratio Rk1 of direct k-fold ionization cross section (σ k) to direct single ionization cross section (σ 1) of Ne impacted by Cq+ (q = 1–3) ions in an energy range of 10 keV/u–500 keV/u are measured in this work. The experimental data are compared with the results from our multi-electron classical over-barrier ionization (ME-COBI) model, showing that the model can give a good estimate to the experimental data.

Electron impact single and double ionization cross sections of Hg atoms

Theoretical studies of electron impact single and double ionization cross sections of Hg atoms have been performed in the binary encounter approximation. Direct double ionization has been investigated in the modified double binary encounter model. Accurate expression of σ ΔE cross section for energy transfer ΔE and the Hartree–Fock velocity distributions for the target electrons have been used throughout the calculations. The present results of single ionization cross sections are in good agreement with the experimental observations. In the case of double ionization, the theoretical cross sections for direct double ionization show overall satisfactory agreement with the experiment. The calculated single and direct double ionization cross sections substantiate the viewpoint that only the direct processes lead to double ionization of Hg and there is no role of Auger effect concerning the measured data of double ionization.

Role of ionization-excitation processes in the cross section for direct ionization of heavy atomic ions by electron impact

The contribution to the ionization cross section of ionization-excitation processes by electron impact is usually negligibly small for low- and medium-$Z$ elements. We demonstrate here, however, that for heavy atomic ions with the outermost shell being $nd$ $(n=4,5)$ the ionization-excitation processes play an evident role in the ionization cross section. For the $4{s}^{2}4{p}^{6}4{d}^{10}$ ground level of ${\mathrm{Gd}}^{18+}$, the ionization-excitation cross section due to the excitation of levels in the $4{s}^{2}4{p}^{6}4{d}^{8}4f$ configuration is comparable to the direct $4p$ and $4s$ ionization cross sections of ${(4{s}^{2}4{p}^{5}4{d}^{10})}_{1/2}$ and ${(4s4{p}^{6}4{d}^{10})}_{1/2}$. The total ionization cross section will be underestimated by 15% without including the contribution from ionization-excitation processes. This is a general conclusion for heavy atomic ions, which is verified by taking Pd-like ions of ${\mathrm{Sn}}^{4+},{\mathrm{Ba}}^{10+},{\mathrm{Nd}}^{14+},{\mathrm{Tb}}^{19+},{\mathrm{Yb}}^{24+}$, and ${\mathrm{W}}^{28+}$ as examples. The role of ionization-excitation processes can be understood from the overlapping of the wave functions between the $4d$ and $4f$ orbitals.

Absolute measurements of chlorine Cl+ cation single photoionization cross section

The photoionization of Cl+ leading to Cl2+ was measured in the photon energy range of 19.5–28.0eV. A spectrum with a photon energy resolution of 15meV normalized to absolute cross-section measurements is presented. The measurements were carried out by merging a Cl+ ion beam with a photon beam of highly monochromatic synchrotron radiation at the Advanced Light Source at Lawrence Berkeley National Laboratory. The measured photoionization of Cl+ consists of several autoionization resonances surperimposed on the direct photoionization signal. Most of the prominent resonances are assigned to members of Rydberg series originating from the singlet ground state and from metastable triplet levels within the ground-state configuration of Cl+. The direct ionization cross section is no larger than 12Mb.

Electron-impact ionization of Se2+

Absolute cross sections for electron-impact single and multiple ionization of Se2+ ions were measured using the dynamic-crossed-beams technique over the energy range from the ionization threshold to 500 eV and compared to direct ionization cross sections predicted by the Lotz semi-empirical formula. The single-ionization cross section appears to be dominated by excitation-autoionization of the 4s and 3d inner subshells. The double-ionization measurements are reasonably well accounted for by the semi-empirical fitting formula of Shevelko et al that includes both direct double ionization of the outer 4p subshell and single ionization of the 3p and 3d subshells followed by autoionization. In addition to direct triple ionization, 3p and 3s ionization followed by double autoionization and excitation of the 3p and 3s subshells followed by triple autoionization likely contribute to the triple ionization cross section.

Electron-impact ionization of Se3+

Absolute electron-impact single and multiple ionization cross-section measurements for Se3+ ions were performed using the dynamic-crossed-beams technique over the energy range from the ionization threshold to 1 keV. The experimental single ionization cross section for Se3+ shows evidence of significant contributions of 4s–nl (n ≥ 6) and 3d–nl (n ≥ 4) inner-shell excitation-autoionization. The cross sections for double and triple ionization are dominated by ionization-autoionization and ionization-double autoionization of a 3p or 3s electron, respectively. The measurements were compared with the direct ionization cross sections predicted by the Lotz semi-empirical formula for direct ionization. The maximum values of cross sections for single, double, and triple ionization are compared.

Ionization cross-sections for positron collisions with N2

The total and direct ionization cross-sections for positron impact on molecular nitrogen have been measured in the energy range from 5 eV to 850 eV. The results are compared with other experimental and theoretical determinations.

Electron-impact ionization of moderately charged xenon ions

Absolute and energy scanned cross sections for the electron-impact single ionization of Xeq + (q = 10–17) atomic ions are measured by employing a crossed-beams technique in the collision energy range from threshold to 1000 eV. Direct ionization and indirect excitation-autoionization cross sections are calculated using a configuration-average distorted-wave method for the ground and first same parity excited configuration of the Xeq + (q = 10–17) atomic ions. Both the crossed-beams experiment and distorted-wave theory show a steady decrease in the ionization cross section as the charge of the ion increases. The disagreements between experiment and theory are partly attributed to the many metastable levels of ground and excited configurations found in the ion beam.