Experimental Ionization Cross Sections Research Articles

Electron interaction with NFx (x = 1–3)

In this theoretical study on electron scattering with NFx (x = 1–3) targets in the energy range spanning from the ionisation threshold (I) to 2000 eV, we quantify various electron-driven molecular processes and report corresponding cross-sections. This study is prompted by the discrepancy between theoretical and experimental ionisation cross-sections ( Q ion ) for NFx (x = 1–3), containing highly electronegative fluorine, and the paucity of elastic and total ( Q tot ) cross-sections for these significant plasma relevant molecules. The quantum spherical complex optical potential method is used to derive elastic ( Q el ) and total ( Q tot ) cross-sections. To compute the Q ion , we employed a semi-empirical method, Complex Scattering Potential – ionisation contribution (CSP-ic). Present findings are compared with existing results available in the literature. This work is the first study to report Q el and Q tot for the two radicals NF and NF2.

Electron interaction with NFx (x = 1–3)

In this theoretical study on electron scattering with NFx (x = 1–3) targets in the energy range spanning from the ionisation threshold (I) to 2000 eV, we quantify various electron-driven molecular processes and report corresponding cross-sections. This study is prompted by the discrepancy between theoretical and experimental ionisation cross-sections ( Q ion ) for NFx (x = 1–3), containing highly electronegative fluorine, and the paucity of elastic and total ( Q tot ) cross-sections for these significant plasma relevant molecules. The quantum spherical complex optical potential method is used to derive elastic ( Q el ) and total ( Q tot ) cross-sections. To compute the Q ion , we employed a semi-empirical method, Complex Scattering Potential – ionisation contribution (CSP-ic). Present findings are compared with existing results available in the literature. This work is the first study to report Q el and Q tot for the two radicals NF and NF2.

The Astrochemistry Low-energy Electron Cross-Section (ALeCS) database

Context. Electron–molecule interaction is a fundamental process in radiation-driven chemistry in space, from the interstellar medium to comets. Therefore, knowledge of interaction cross sections is key. There have been a plethora of both theoretical and experimental studies of total ionization cross sections spanning from diatomics to complex organics. However, the data are often spread over many sources or are not public or readily available. Aims. We introduce the Astrochemistry Low-energy Electron cross-section (ALeCS) database. This is a public database for electron interaction cross sections and ionization rates for molecules of astrochemical interest. In particular, we present here the first data release, comprising total ionization cross sections and ionization rates for over 200 neutral molecules. Methods. We include optimized geometries and molecular orbital energies at various levels of quantum chemistry theory. Furthermore, for a subset of the molecules, we have calculated ionization potentials. We computed the total ionization cross sections using the binary-encounter Bethe model and screening-corrected additivity rule, and we computed ionization rates and reaction network coefficients for molecular cloud environments. Results. We present the cross sections and reaction rates for >200 neutral molecules ranging from diatomics to complex organics, with the largest being C14H10. We find that the screening-corrected additivity rule cross sections generally significantly overestimate experimental total ionization cross sections. We demonstrate that our binary-encounter Bethe cross sections agree well with experimental data. We show that the ionization rates scale roughly linearly with the number of constituent atoms in the molecule. Conclusions. We introduce and describe the public ALeCS database. For the initial release, we include total ionization cross sections for >200 neutral molecules and several cations and anions calculated with different levels of quantum chemistry theory, the chemical reaction rates for the ionization, and network files in the formats of the two most popular astrochemical networks: the Kinetic Database for Astrochemistry, and UMIST. The database will be continuously updated for more molecules and interactions.

Computed corrections for recollision effects on ionization and x-ray production cross sections obtained in positron-impact studies

The accurate experimental measurement of inner-shell ionization cross sections induced by positrons near the threshold energy is of great significance both for theory and practical application. So far, there are few published experimental data in this field, most of which involve a slow positron beam device and acceleration fields to achieve the desired collision energies. Most previous experimental studies, in which a negative acceleration potential was applied to the target, neglected to consider how backscattered positrons might be accelerated back to the target, generating surplus characteristic x-rays and inflating the cross-section estimates derived therefrom. Using Geant4 and Ansys Maxwell to simulate charged-particle transport, the experimental collision process in the negative high-voltage electromagnetic field of the target chamber is simulated realistically in this paper. Through the simulation process, the effect of the negative high-voltage electromagnetic field on backscattered positrons, hence on ionization and characteristic x-ray production cross sections, is obtained. We then estimate corrections for previously published experimental Ti K-shell ionization cross sections and Ag-Lαβγ, Bi-Mαβ, Pb-Mαβ x-ray production cross sections obtained by positron impact. In addition, we use simulation to demonstrate that such undesired effects can be suppressed by adjusting the size of the target and the tilt angle between the target normal and the positron beam axis.

Numerical investigation of a high-pressure gas medium preionization by runaway electrons

A comparative simulation of the generation and acceleration of runaway electrons in the discharge gap during the initiation of the discharge by nanosecond and subnanosecond pulses is carried out. We used a numerical model based on the PIC-MCC method. Calculations were carried out for N2 6 atm pressure. Numerical simulation of a formation process of the electron avalanche initiated by an electron field-emitted from the top of the cathode microspike was carried out taking into account the motion of each electron in the avalanche. Characteristic runaway electron trajectories, runaway electron energy gained during the motion through the discharge gap, times required for runaway electrons to reach the anode were calculated. We compared our results with calculations using well-known differential equation of electron acceleration using braking force in Bethe approximation. We solved this equation also for braking force based on real (experimental) ionization cross section. The reasons for the discrepancy in the calculation results are discussed.

Evaluation of accurate uncertainty of measurement of L subshell ionization cross-sections

Experimentally, X-ray production cross-sections are measured, but ionization cross-sections are calculated theoretically. The uncertainty of the measured L X-ray production cross-section is mainly statistics and detector-efficiency driven. But the experimental ionization cross-section involves many other factors because the relationship between the production and ionization cross section involves various atomic parameters and thus its uncertainty equation is complex. Consequently, determining the measurement uncertainty in L subshell ionization cross-section is always difficult. We have studied this issue in the simplest way, where the rule of weighted propagation of relative uncertainty is utilized. We notice that larger uncertainties are involved in atomic parameters relevant to L1 (2s1/2) subshell than those associated with the other two L2 (2p1/2) and L3 (2p3/2) subshells. Hence, comparison between theory and experiment would give higher emphasis on L2 and L3 subshell ionization cross sections. We believe this work makes us aware that the appropriate uncertainty evaluation is extremely important for providing the right judgement on the data.

High energy PIXE: New experimental K-shell ionization cross sections for silver and gold and comparison with theoretical values from ECPSSR/RECPSSR models

In high energy PIXE, a high energy ion beam, up to several tens of MeV, is used to analyze heavy elements in a thick sample. The knowledge of the interaction cross-section with the atoms is essential to perform quantitative analysis. As measured data in this range of energy are scarce, an experimental campaign was conducted at the C70 cyclotron of the GIP ARRONAX to measure the K-X-ray production cross-sections for silver and gold with proton energies 30, 42, 54 and 68 MeV. The detector efficiency, the beam intensity, and the target thickness were measured accurately to produce precise data. To convert the K-X-ray production cross-sections into the K-shell ionization cross-sections, an average value of the K-shell fluorescence yield, calculated from a compilation of existing data, was used. The outcomes of the campaign are in good agreement with the few existing data and the theoretical prediction of the RECPSSR model.

Experimental and theoretical L-shell ionization cross sections of heavy atoms by impact of Si ions

We present a theoretical and experimental study of the subshell resolved L-shell ionization of relativistic targets such as 73Ta, 78Pt, 90Th, and 92U. The measurements of x-ray production cross sections by (84–140 MeV) Si+q ions (q = 8; 12), were held at the Inter-University Accelerator Centre of New Delhi. Multiple-hole fluorescence and Coster-Kronig yields were used to obtain the Li(i = 1–3) ionization cross sections from the measured x-ray production cross sections of Lℓ, Lα, and Lβ, Lη, and Lγ lines. The experimental results are compared with ab initio theoretical calculations by means of the shell-wise local plasma approximation (SLPA). This model uses the quantum dielectric formalism to obtain the total ionization cross sections from an initial ground state. The wave functions and binding energies of the different targets were obtained by solving the fully relativistic Dirac equation using the HULLAC code package. These calculations are based on first order perturbation theory with a central field, including Breit interaction and quantum electrodynamics corrections. The present SLPA ionization cross sections of the L-shell are found to be independent of the charge state of the Si ions. The experimental observations display also quite similar character if the correct mean projectile charge state inside the target is used for including the multiple ionization effect during ion-solid collisions. A general good agreement between the experimental measurements and full theoretical calculations supports the reliability of present results. The comparison also includes the well-known ECPSSR and ECUSAR semi empirical approximations. We noted that the ECUSAR results agree well with the SLPA, while the ECPSSR cross sections are rather low.

Electron-impact ionization cross section of formic acid

Experimental electron-impact ionization cross sections of one of the simplest carboxylic acids, formic acid, are presented. The molecular target was studied in two independent experimental setups applying different methods: the total ion collection and the electron–impact mass spectrometry methods. Experimental data were taken at incident electron energies ranging from ionization threshold [11.31 eV; J.C. Traeger, Int. J. Mass Spectrom. Ion Process. 66, 271 (1985)] to 76 eV using a total cross section of 4.0 Å2 at 70 eV for the total ion collection method. In the spectrometric method the four main cations COH+, HCOOH+, COOH+ and CO+ are found to contribute to about 97% of the total ionization cross section.Graphical abstract

Ionization cross sections of the Au L subshells by electron impact from the L3 threshold to 100 keV

We measured the cross sections for Au Lα, Lβ, Lγ, Lℓ and Lη x-ray production by the impact of electrons with energies from the L3 threshold to 100 keV using a thin Au film whose mass thickness was determined by Rutherford Backscattering Spectrometry. The x-ray spectra were acquired with a Si drift detector, which allowed to separate the components of the Lγ multiplet lines. The measured Lα, Lβ, , L, , , and Lη x-ray production cross sections were then employed to derive Au L1, L2 and L3 subshell ionization cross sections with relative uncertainties of 8%, 7% and 7%, respectively; these figures include the uncertainties in the atomic relaxation parameters. The correction for the increase in electron path length inside the Au film was estimated by means of Monte Carlo simulations. The experimental ionization cross sections are about 10% above the state-of-the-art distorted-wave calculations.

Evaluation of relative electron ionization cross-sections for some oxides and oxyacid salts.

The total or relative cross-sections for the ionization of polyatomic molecules by electron ionization are essential data in a wide range of applications. In addition to the total ionization cross-sections, relative ionization cross-sections are sometimes also required. Accurate values of electron ionization cross-sections of high-temperature vapor species are of importance in mass spectrometric investigations. So the need for experimental ionization cross-section data for high-temperature vapor species is vital. Measurements were performed by high-temperature Knudsen effusion mass spectrometry with a MS-1301 mass spectrometer. Vaporization was carried out using molybdenum or tungsten effusion cells containing samples of pure Au and CeO2 . The vapor compositions over the CeO2 -Mo and CeO2 -W systems were determined by Knudsen effusion mass spectrometry. The method of ion currents comparison was used to measure partial pressures. The enthalpies of the reactions under study were calculated using the third law procedure. The standard formation enthalpies of cerium molybdates and tungstates were determined from the relative ionization cross-sections of molybdenum, tungstate, cerium, and cerium salts. Cross-sections were calculated by different methods: simple adding atomic cross-sections with correction factor, using known ratios of complex ions, adding oxide cross sections. The traditional approach to determination of ionization cross-sections exploiting the values of atomic cross-sections and the additivity rules as well as literature experimental data was used. The enthalpies of formation of gaseous cerium molybdates and tungstates were evaluated by measurements of the reaction enthalpies for several independent reactions. The best agreement was obtained with the data from the analysis of experimentally determined relative ionization cross-sections, and using ionization cross-sections of polyatomic molecules recommended by Drowart et al. The widely used additivity method, in spite of the additional corrections introduced, gives the worst convergence of the values under consideration.

Production of projectile and target K-vacancy in near-symmetric collisions of 60–100 MeV Cu9+ ions with thin Zn target**Project supported by the Joint Funds of the National Natural Science Foundation of China (Grant No. U1332122).

We report studies on both target and projectile K-shell ionization by collisions of Cu9+ ions on the thin Zn target in the energy range of 60–100 MeV. In this work, the relative ratio for the production of the target to projectile K-vacancy is measured. The result shows that it almost remains stable over this energy range and has good consistency with the predictions by vacancy transfer via the 2pσ–1sσ rotational coupling, which gives experimental evidence for K-vacancy sharing between two partners. Furthermore, the discussion for comparisons between the experimental ionization cross sections and the possible theoretical estimations is presented. These comparisons suggest that the experimental data agree well with those predicted by the Binary–Encounter approximation (BEA) model but are not in good agreement with the modified BEA calculations. It allows us to infer that the direct ionization (and/or excitation) is of importance to initial K-vacancy production before 2pσ–1sσ transitions in the present collision condition.

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.

Ionization and electron capture cross sections for single and multiple electron removal from H2O by Li3+ impact

SynopsisWe report experimental and theoretical ionization and single electron capture cross sections for single, double and triple electron removal from H2O by Li3+ with energies ranging from 0.75 to 5.8 MeV. Single electron removal is dominant for ionization, while double electron removal is dominant for electron capture. For the one- and two-electron removal cases, the calculations based on time-dependent orbital propagation in density functional theory show good agreement with experiment for the two collision channels studied.

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.

High‐temperature mass spectrometric determinations of relative ionization cross‐sections of gaseous TiO, TiO2, VO, VO2, YO, HfO and GeO molecules

Accurate values of electron ionization cross-sections of high-temperature vapor species are of importance in mass spectrometric investigations as well as in a variety of other fields. However, the present experimental techniques are subject to many inherent difficulties or are limited in applicability and so far have yielded relatively few reliable values. Theoretical calculations are not sufficiently accurate. So the need for experimental ionization cross-section data for these species is very important. Measurements were performed by high-temperature Knudsen effusion mass spectrometry with a MS-1301 mass spectrometer. Vaporization was carried out using molybdenum effusion cells containing samples of pure Au, V2O3, Ti3O5, GeO2 and Y2O3-HfO2 systems. The VO, VO2, TiO, TiO2, YO, HfO and GeO vapor species were identified over the samples studied. The partial pressures of Au and the oxides were measured by a complete isothermal vaporization method. The relative ionization cross-sections σ(Au)/σ(i) were determined for the gaseous oxides studied. The results were compared with the literature data. The inapplicability of the 'additivity rule' is shown. This finding is in good agreement with a tendency to changing ionization cross-sections in the Periodic Table row of gaseous M, МО and МО2 species.

Quantum-mechanical predictions of electron-induced ionization cross sections of DNA components

Ionization of biomolecules remains still today rarely investigated on both the experimental and the theoretical sides. In this context, the present work appears as one of the first quantum mechanical approaches providing a multi-differential description of the electron-induced ionization process of the main DNA components for impact energies ranging from the target ionization threshold up to about 10 keV. The cross section calculations are here performed within the 1st Born approximation framework in which the ejected electron is described by a Coulomb wave whereas the incident and the scattered electrons are both described by a plane wave. The biological targets of interest, namely, the DNA nucleobases and the sugar-phosphate backbone, are here described by means of the GAUSSIAN 09 system using the restricted Hartree-Fock method with geometry optimization. The theoretical predictions also obtained have shown a reasonable agreement with the experimental total ionization cross sections while huge discrepancies have been pointed out with existing theoretical models, mainly developed within a semi-classical framework.

Preliminary Measurement of the K-Shell Ionization Cross Sections of Ti by Positron Impact in the Low Energy Region

Measurements were performed of K-shell ionization cross sections of Ti element by 10∼30 keV positron impact using the thick-target method. The effects of multiple scattering of incident positron and from bremsstrahlung photons and annihilation photons with the thick-target method are discussed with the Monte Carlo code PENELOPE. Meanwhile, the Monte Carlo method is also applied to determine the detection efficiencies of X- and γ-ray detectors. Our experimental K-shell ionization cross sections for Ti element are compared with the distorted-wave Born approximation (DWBA) theoretical predictions, and it is found that the agreement of the experimental data and theoretical values is good and this indicates that the experimental method adopted in this study is applicable.

Near-threshold electron-impact doubly differential cross sections for the ionization of argon and krypton

We present normalized doubly differential cross sections (DDCS's) for the near-threshold, electron-impact single ionization of argon and krypton, similar to those taken earlier for Ne and Xe [Yates et al., J. Phys. B 42, 095206 (2009)]. The Ar measurements were taken at incident energies of 17, 18, 20, and 30 eV while the Kr measurements were taken at 15, 16, 17.5, and 20 eV. The DDCS scattering angles range from 15${}^{\ifmmode^\circ\else\textdegree\fi{}}$ to 120${}^{\ifmmode^\circ\else\textdegree\fi{}}$. The differential data are initially normalized to available experimental cross sections for excitation of the ground ${\mathit{np}}^{6}$ to the ${\mathit{np}}^{5}(n+1)s$ excited states of the noble gas and, after integration, to well-established experimental total ionization cross sections of Rapp and Englander-Golden [J. Chem. Phys. 43, 1464 (1965)].

EDDIX--a database of ionisation double differential cross sections

The use of Monte Carlo track structure is a choice method in biophysical modelling and calculations. To precisely model 3D and 4D tracks, the cross section for the ionisation by an incoming ion, double differential in the outgoing electron energy and angle, is required. However, the double differential cross section cannot be theoretically modelled over the full range of parameters. To address this issue, a database of all available experimental data has been constructed. Currently, the database of Experimental Double Differential Ionisation Cross sections (EDDIX) contains over 1200 digitalised experimentally measured datasets from the 1960s to present date, covering all available ion species (hydrogen to uranium) and all available target species. Double differential cross sections are also presented with the aid of an eight parameter functions fitted to the cross sections. The parameters include projectile species and charge, target nuclear charge and atomic mass, projectile atomic mass and energy, electron energy and deflection angle. It is planned to freely distribute EDDIX and make it available to the radiation research community for use in the analytical and numerical modelling of track structure.