Direct Ionization Processes Research Articles

Electron-impact ionization for Ne[formula omitted] and Ne[formula omitted]

Ionization cross sections are studied for energy levels of the ground configurations of the Ne3+ and Ne4+ ions. The distorted wave (DW) approximation is used to analyze experimental data. The scaled DW cross sections are used to explain measurements for the Ne3+ ion. Study includes analysis of contributions from the direct and indirect ionization processes. Convergences of excitation-autoionization (EA) channels are estimated by analyzing excitations up to shells with the principal quantum number n⩽20. This study shows that the EA process provides from ∼10% to 20% for the energy levels of the ground configurations of the Ne3+ and Ne4+ ions.

Electron-impact double ionization cross sections and rate coefficients for Be-like ions: B+ to Ne6+

We report a detailed analysis of the electron-impact double ionization process for Be-like ions, spanning charge states from B+ to Ne6+. We considered both direct and indirect double ionization processes. For direct double ionization, we calculated the cross sections by considering three different processes: ionization-ionization (II), excitation-ionization-ionization (EII), and ionization-excitation-ionization (IEI). Our results show excellent agreement between the measured cross sections for Be-like-B and Be-like-O ions. The agreement is slightly weaker for Be-like-Ne ions when we take the potential of ionized ions in the distorted-wave method. For other ions for which experimental data are not available, our calculations should provide the missing data. We have summarized the trends of electron-impact double ionization cross sections with atomic number for isoelectronic sequences. Finally, we have provided the Maxwellian and non-Maxwellian rate coefficients for these Be-like ions. The data obtained should be useful for both equilibrium and non-equilibrium charge-state distributions in astrophysical research.

Excitation Dynamics of the 5p^6 Auger Spectra in Ba + e^– Collisions

The ionization of the 5p6 subshell in Ba atoms has been studied in an electron-impact energy range 23–105 eV by measuring the Auger spectra arising from the decay of the 5p5nln′l′ states. The energy dependences of the ionization cross-section have been obtained for nine states in the 5p55d2, 5p55d6s and 5p56s2 configurations. The analysis of the behavior of the cross-sections made it possible to determine the role of direct and indirect ionization processes involved in the 5p6 ionization of the Ba atom.

Electron-impact single ionization for the N[formula omitted] and N[formula omitted] ions

Electron-impact ionization cross sections are investigated for the ground levels of N2+ and N3+ ions. Additionally, the long-lived energy levels of the N2+2s2p2 and N3+2s2p configurations are analyzed. Study includes both direct and indirect ionization processes. Contribution of the excitation–autoionization (EA) channels to the ionization cross sections is approximately 15% to 23% for both ions. Convergence of the EA cross sections is analyzed by considering excitations up to shells with the principal quantum numbers n⩽20. The obtained results are compared to available experimental data for the N2+ and N3+ ions.

Electron-impact single ionization for the Fe2+ ion

Electron-impact ionization was analyzed for all 34 levels of the ground configuration of the Fe2+ ion. Both direct and indirect ionization processes are included in the study. For the direct process, the potential of the ionized ion was used to explain experimental data. For the indirect process, electron-impact excitation with subsequent autoionization was considered. The convergence of the cross sections of the indirect process was determined by considering the excitations up to shells with the principal quantum number n ⩽ 20. The main contribution to the indirect process was determined by the 3p → 3d excitations. The scaled distorted-wave approximation was applied to explain measurements for the Fe2+ ion. Scaling of the cross sections lowers values by ~35% at the peak for the ground level and ~50% for the highest level of the ground configuration of the Fe2+ ion. The experimental value for the single ionization threshold was used to provide better agreement with the measurements for the cross sections. This led to the cross sections for the levels of the ground configuration diminishing in size by 15–25%. The direct process contributes ~70% and ~50% to the total ionization from the lowest and highest levels of the ion, respectively.

Dynamics of the H2 levels lying between the third and fourth dissociation thresholds (132 500 – 139 000 cm− 1)

The absorption spectrum of H2 at room temperature, was recorded at high resolution at the BESSY II synchrotron radiation storage ring in the 132500 – 139000 cm−1 energy range, allowing the study of the dynamics of the excited levels (npσ 1Σu+ and npπ 1Πu±) located between the H(n = 3) + H(n = 1) and H(n = 4) + H(n = 1) thresholds. Ionization anddissociation were found to compete for levels with n ≤ 8. In most cases, for the levels with n > 8, ionization is the only decay channel. The relative importance of the direct ionization, direct dissociation, autoionization and predissociation processes was evaluated for the whole energy range. From the different detected channels, and from the comparison with theoretical approaches, the main continua responsible for the predissociations were assigned to the V and to the B"B¯ states and the H(3p)/H(n = 3) ratios of the dissociation fragments from the 4pπ and 5pπ 1Πu- levels were measured. Molecular fluorescence was observed for few 1Πu- levels and the importance of its contribution is discussed.

Proton Direct Ionization Upsets at Tens of MeV

Experimental mono-energetic proton single-event upset (SEU) cross-sections of a 65 nm low core-voltage static random access memory (SRAM) were found to be exceptionally high not only at low energies (< 3 MeV), but also at energies > 3 MeV and extending up to tens of MeV. The SEU cross-section from 20 MeV protons exceed the 200 MeV proton SEU cross-section by almost a factor of 3. Similarly, mono-energetic neutron cross-sections at 14 MeV are about a factor of 3 lower than the 20 MeV proton cross-section. Thanks to Monte-Carlo (MC) simulations it was determined that this strong enhancement is due to the proton direct ionization process as opposed to the elastic and inelastic scattering processes that dominate the SEU response above 3 MeV in other SRAMs. As shown by means of a detailed energy deposition scoring analysis, however, this does not appear to be caused by the critical charge of the SRAM being lower than the charge resulting from the average proton ionization through the linear energy transfer (LET). On the other hand, this is caused by high-energy δ-rays (> 1 keV) that can deposit their full kinetic energy within the sensitive volume (SV) of a cell despite their range being theoretically much longer than the characteristic size of the SV. Multiple scattering events are responsible for increasing the trajectory path of the δ-rays within the sensitive volume, resulting in a 6 fold increase in the probability of upset with respect to the sole electron ionization.

Towards the ionizing radiation induced bond dissociation mechanism in oxygen, water, guanine and DNA fragmentation: a density functional theory simulation

The radiation-induced damages in bio-molecules are ubiquitous processes in radiotherapy and radio-biology, and critical to space projects. In this study, we present a precise quantification of the fragmentation mechanisms of deoxyribonucleic acid (DNA) and the molecules surrounding DNA such as oxygen and water under non-equilibrium conditions using the first-principle calculations based on density functional theory (DFT). Our results reveal the structural stability of DNA bases and backbone that withstand up to a combined threshold of charge and hydrogen abstraction owing to simultaneously direct and indirect ionization processes. We show the hydrogen contents of the molecules significantly control the stability in the presence of radiation. This study provides comprehensive information on the impact of the direct and indirect induced bond dissociations and DNA damage and introduces a systematic methodology for fine-tuning the input parameters necessary for the large-scale Monte Carlo simulations of radio-biological responses and mitigation of detrimental effects of ionizing radiation.

Contribution of the metastable states to electron-impact single ionization for W7+

In this paper, we present the electron-impact single ionization cross sections for W7+. The level-to-level distorted-wave method implemented in the flexible atomic code (FAC) was used for calculation. Contributions from direct ionization (DI) and excitation-autoionization (EA) processes are taken into account. Comparison between the results of previous experimental measurements and the present calculations show a prominent contribution from metastable states in the W7+ ion. We extended the theoretical analysis including level-to-level calculations from long-lived levels of the [Cd]4f135p6 ground configuration and excited, metastable [Cd]4f145p5, [Cd]4f135p55d and [Cd]4f145p45d configurations. Calculated results are in good agreement with experimental data when, within a statistical model, a 56% fraction of parent ions in the ground configuration is assumed.

Electron-impact ionization of Ar2+

We study electron-impact single-ionization cross-sections for all levels of the ground configuration of the Ar2+ ion using the distorted wave approximation. We investigate the influences of the direct ionization and excitation-autoionization processes on the total single-ionization cross sections. We used corrected energies for the levels of the Ar3+ ion so that the ionization threshold corresponds to the value recommended by National Institute of Standards and Technology (NIST). Radiative damping is taken into account for the ionization cross-sections of the indirect process. Excitations from the 3s and 3p subshells are investigated for the indirect process. We demonstrate that the excitations from the outermost 3p subshell of the ground configuration provide an important contribution to the single ionization cross-sections at low energies. Fairly good agreement with measurements is obtained for the ground level of the Ar2+ ion. We show that the direct ionization provides ~70% at peak of the total single-ionization cross-sections.

Producing Iron Endohedral Fullerene on Electron Cyclotron Resonance Ion Source

An electron cyclotron resonance (ECR) ion source (ECRIS) can generate an available amount of multicharged ions, thus it is not limited for use in the field of accelerator science, but also in medical/biological fields, such as for heavy ion beam cancer treatment and ion engines. The processes of generating multicharged ions are mainly sequential collisions of a direct ionization process by electrons, and have good ion confinement characteristics. By utilizing this confinement property, we have synthesized iron-encapsulated fullerenes, which are supramolecular and can be expected to have various high functions. Fullerenes and iron ions are vaporized from pure solid materials and introduced into the ECRIS together with the support gas. We investigated conditions under which fullerene ions do not dissociate and iron ions are generated so that both can coexist. Generated ions are extracted from the ECRIS and separated by mass/charge with a dipole magnet, and detected with a Faraday cup. This measurement system is characterized by a wide dynamic range. The charge-state distribution (CSD) of ion currents was measured to investigate the optimum conditions for supramolecular synthesis. As a result, a significant spectrum suggesting the possibility of iron-encapsulated fullerenes was obtained. This paper describes the details of these experimental results.

Electron-impact single, double, and triple ionization of B+

Electron-impact single, double, and triple ionizations are investigated for the ${\mathrm{B}}^{+}$ ion. The direct and indirect ionization processes are studied for the single and double ionization. Good agreement with measurements for the single and double ionization is obtained when the scaled distorted-wave cross sections are used to study the collisional ionization and excitation processes. It is shown that ionization-excitation-autoionization and excitation-ionization-autoionization processes have to be introduced to the total double ionization to explain the measurements. The triple ionization is investigated by considering the direct double ionization with the subsequent autoionization.

Extraction of Ps-formation cross-sections from single-centre positron-scattering calculations

Positron scattering on atomic and molecular targets is a two-centre problem due to the possibility of rearrangement (Ps-formation) processes. In certain kinematic regions, single-centre expansion methods can be applied to two-centre scattering problems. In such approaches, however, information about Ps-formation and direct ionization processes are combined into excitation amplitudes of positive-energy pseudostates. In this report we show that Ps-formation amplitudes can be extracted using the total scattering wave function, which we reconstruct from single-centre convergent close-coupling calculations. The method is applied to positron scattering from hydrogen and lithium atomic targets. Excellent agreement with the two-centre theoretical benchmark results show the validity of the proposed technique.

Systematic investigation of the pulsed barrier discharges in flowing and stationary gas: From differences to similarities

Pulsed dielectric barrier discharges in flowing and stationary helium at atmospheric pressure are comparatively studied. The discharges are generated in plane-parallel configuration and driven by positive voltage pulses. The intensified charge coupled device imaging and electrical measurements are used for temporally, spatially, and spectrally resolved discharge emission. The populations of excited states are investigated by optical emission and absorption spectroscopy. The discharge operating in flowing helium is characterized by a single current pulse per each voltage flank and a “residual current peak.” In stationary helium or in flowing helium with low velocity (&amp;lt;10 cm/s), multiple current pulses are observed. The appearance of the subsequent current pulse(s) is related to the disappearance of the “residual current peak.” Besides these differences between the two discharge regimes, some similarities are also registered. The maximum emission intensity is reached at the cathode, even for the subsequent current pulses formed in stationary helium, confirming the generation of the glow and multi-glow discharges. Also, the most important similarities between the discharges working in stationary helium and in flowing gas mixture, in relation to flowing helium mode, are discussed. Finally, the subsequent current pulses formed in multi-glow discharges are due to re-ignition of the discharge dominantly by direct electronic excitation and ionization processes, not by Penning ionization, because of the decrease in metastable atoms density and their lifetime. In addition, the metastable lifetime reaches a critical value, for which the discharge transits from glow to multi-glow mode, when the gas velocity or its composition changes.

Ionization of Hydrogen-Like Atoms by Proton Impact

Direct ionization processes of hydrogen-like atoms by proton impact are investigated by Born theory. The triple differential cross sections (TDCSs) and the quantities of physical interest in the process are calculated for ionization of H, He+, Li2+, and Be3+ ground-state atoms, using the first- and second-Born approximations. The closure relation is employed to come over the consideration of the intermediate states. The calculations are carried for fixed scattering angles and ejected electron energies at the impact energies 75, 200, and 500 keV. The influence of each of the incidence energy and the atomic number of the target atom on the TDCSs is studied. The contribution of the second-Born term to the cross section is illustrated.

Theoretical investigation of electron-impact multiple ionization of O ii-iv ions

The electron-impact double ionization (DI) and triple ionization (TI) for O ii-iv ions are investigated using multistep approaches within the framework of many-body perturbation theory. The indirect DI and TI are treated as the single ionization of an inner-shell $1s$ electron followed by the single and double Auger decays, respectively. The knockout mechanism is employed to deal with the direct ionization process. The present approaches reproduce the available experimental cross sections for the DI of O ii-iv ions with much better accuracy compared to previous calculations. The TI cross sections of the O ii ion are calculated, which are in good agreement with the recent experimental data [Lecointre et al., J. Phys. B 46, 205201 (2013)]. It is found that the electron correlation effect and the energy distribution of the intermediate electron, which is taken as the incident electron of the subsequent knockout process, play an important role in the multistep treatment of the multiple ionization process.

Absolute cross sections for production of molecular dications by electron impact

Molecular dications are important metastable species present in a variety of environments including, but not restricted to, planetary atmospheres and the human body when submitted to radiation. On the other hand, symmetric molecular dications cannot be separated from the cation corresponding to the molecule broken in half (e.g. and N+) in stantard mass spectrometers. The DETOF technique allows one to identify the symmetric molecular dication, obtaining absolute cross sections related to their production. Results for dications of molecular nitrogen, molecular oxygen, ethylene and benzene are discussed here, giving emphasis on the mechanisms behind their production. It is seen that, while most metastable molecular dications result from a combination of direct double ionization and Auger-like processes, and represent so-called pure cases, where the dication is produced only by, respectively, double direct ionization or post-collisional Auger-like decay.

Inner-shell ionization of W25+

SynopsisInner-shell ionization by electron impact is investigated for the W25+ ion. Decay of inner-shell vacancy through Auger transitions is analyzed. Contribution of direct ionization process to double and triple ionization is presented.

Parametrization of energy sharing distributions in direct double photoionization of He

We present experimental results on the characteristic sharing of available excess energy, ranging from 11–221 eV, between two electrons in single-photon direct double ionization of He. An effective parametrization of the sharing distributions is presented along with an empirical model that describes the complete shape of the distribution based on a single experimentally determinable parameter. The measured total energy sharing distributions are separated into two distributions representing the shake-off and knock-out parts by simulating the sharing distribution curves expected from a pure wave collapse after a sudden removal of the primary electron. In this way, empirical knock-out distributions are extracted and both the shake-off and knock-out distributions are parametrized. These results suggest a simple method that can be applied to other atomic and molecular systems to experimentally study important aspects of the direct double ionization process.

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.