Electron-impact Cross Sections Research Articles

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 single and double ionization of Fe12+

The cross sections of electron-impact single and double ionization of Fe12+ are investigated using the distorted-wave (DW) method and two-step approximate formula based on the knockout (KO) mechanism. Both direct and indirect processes are studied in single and double electron-impact ionization. The better agreements between our theoretical results and experiment measurements are obtained contrasted to earlier theoretical works. Adequate indirect ionization channels with larger configurations space are used in our calculation, which is significant for understanding of the mechanism of single and double electron-impact ionization for highly charge ion Fe12+.

Effect of plasma screening on electron impact excitation and ionization of Fe16+ in a dense environment

ABSTRACT Electron impact excitation and ionization with atoms and ions within a dense plasma are fundamental microscopic processes that determine the ionization balance, physical properties (such as electron conductive opacity and thermal conductivity) and plasma formation and dynamics. While collision cross-sections and rates are well studied in dilute systems, similar investigations are scarce for dense plasmas under stellar interior conditions using an appropriate plasma-screening potential. Here we investigate the plasma-screening effect on the electron impact excitation and ionization cross-sections, effective collision strengths, and rate coefficients within plasmas under stellar interior conditions in a mass density range of 1–15.748 g cm−3 and a temperature range of 200–1000 eV. These investigations were carried out using our recently developed plasma-screening model, taking Fe16+ as an example. The results show that the cross-sections of the electron impact excitation are generally decreased, whereas they are always significantly increased for the collision ionization due to the plasma screening. In a plasma at a temperature of 200 eV and density of 15.748 g cm−3, the plasma screening causes a decrease in the excitation cross-section of 36 per cent for the dipole-allowed transition $2\mathrm{ s}^22\mathrm{ p}^6~^1\mathrm{ S}_0 \rightarrow 2\mathrm{ s}^22\mathrm{ p}^53\mathrm{ d}~^1\mathrm{ P}^o_1$ and of 50 per cent for the dipole-forbidden transition $2\mathrm{ s}^22\mathrm{ p}^6~^1\mathrm{ S}_0 \rightarrow 2\mathrm{ s}^22\mathrm{ p}^53\mathrm{ d}~^3\mathrm{ D}^o_1$. However, the collision ionization cross-section of a 2p electron from the ground level of Fe16+ is increased by 500 per cent and 100 per cent under an incident electron energy of 1500 and 10 000 eV, respectively. This results in the rate coefficient increasing by a factor of 18.5 at a temperature of 200 eV and density of 15.748 g cm−3.

The efficient calculation of electron impact ionization cross sections with effective core potentials.

A black box Binary Encounter Bethe (BEB) with an effective core potential (ECP) procedure is implemented, which facilitates the efficient calculation of electron impact ionization cross sections for molecules that include heavy atoms. This is available in the Quantemol electron collisions software, a user friendly graphical user interface to the UKRMol+ codes. Tests were performed for the following series of molecules: CF4, CCl4, CBr4, CI4, and CAt4; CH4, SiH4, GeH4, and SnH4; PH3, PF3, and PCl3; SiCl4 and BCl3; and CH3Br and CF3I. Use of an ECP generally raises the predicted ionization cross section at lower energies leading to improved agreement with experiment compared to all electron calculations for BEB cross sections. Scaling BEB cross sections by the polarizability of the target molecule is shown to give somewhat erratic results, which do not always provide closer agreement with the measured cross sections.

Calculation of gas-kinetic radius and van der Waals coefficient from electron scattering cross-sections

The gas-kinetic radius (r) and van der Waals coefficient (b) are two usable quantities for characterizing the motion of any molecules in the gas phase. In this article, comprehensive calculations of r and b of various environmentally sensitive molecules like SF6, NF3, GeF4, Si2H6 and Cl2O are reported. In the present study, an analytical approach for calculations of r and b was used from the electron impact cross-sections data of these molecules. In general, the reported values of r and b shows good agreement with the previous theories and experiments.

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

Partial ionization cross sections of Silicon by Electron impact Ionization process

Electron-impact single-ionization cross sections of Si by electron impact have been solved theoretically for the full range of kinematics and collision geometries of practical interest by S.P. Khare theoretical model. The corresponding partial and total ionization cross sections have also been derived in the energy range varying from ionization thresholds to 6000 eV. Comparison of the evaluated partial and total ionization cross sections is made with the experimental and theoretical data wherever available. Objective:Our objective is to find the partial and total ionization cross sections Silicon atom and its fragmentation ion at different energy levels and analysis of results with other available data. Method: In this present work we have measured partial ionization cross sections of Silicon atom using semi-empirical formalism of Jain & Khare due to electron impact at incident electron energy from ionization threshold to 6000 eV. Findings: Comparison of the evaluated partial and total ionization cross sections is made with the experimental and theoretical data wherever available. A good agreement is observed when we compared our data for electron impact ionization cross section for Silicon and its fragment ions. Also some disagreement is found between our data and other available data. Our results are higher for Si2+, Si4+, Si5+ and Si6+ fragment ions. For Silicon atom, good agreement between theory and experiment is achieved. Novelty: The total ionization cross-sections by electron impact of atoms are required in the study of plasma diagnostics, astrophysical and fusion applications, radiation physics, mass spectrometry, ionization in gas discharge, modeling of fusion plasmas, modeling of radiation effects for both materials & medical research and astronomy. We have calculated partial and total ionization cross section for higher energy range i.e from threshold to 6000 eV, which have not been done by other researchers. Keywords: Ionization; cross section; electron impact

Electron impact ionization cross-sections of Chromium

The theoretical model, developed by S.P. Khare, has been modified to evaluate the total cross sections for ionization of Chromium atom due to electron impact at incident electron energy from ionization threshold to 6000 eV. The various calculated cross sections are in remarkable agreement with available experimental data and other theoretical cross sections. For Chromium atom, good agreement between theory and experiment is achieved. Objective: Our objective is to find the partial and total ionization cross sections for Chromium atom and its fragmentation ion at different energy levels and interpretation of results with other available data. Method:-The theoretical semi-empirical model, developed by S.P. Khare, has been remodeling to evaluate the total cross sections for ionization of Chromium atom due to electron impact at incident electron energy from ionization threshold to 6000eV. Findings: A good agreement is observed when we compared our data for electron impact ionization cross section for Cr6+, Cr7+, Cr8+ and Cr10+ fragment ions. Also some disagreement is found between our data and other available data. Our results are higher for Cr7+, Cr8+ fragment ions. For Chromium atom, good agreement between theory and experiment is achieved. Novelty: Electron impact ionization is engaged to produce positive and negative ions in many areas of physics and chemistry such as plasma studies, fusion modeling, radiation physics and gas discharges to more abstract applications such as astrophysical applications and modeling of planetary atmospheres. Keywords: cross- section; impact; spectroscopy

Application of molecular convergent close-coupling cross sections in a collisional radiative model for the triplet system of molecular hydrogen

Collisional radiative (CR) models for molecular hydrogen are of high relevance for performing qualitative and quantitative analysis of excited-state population densities measured in plasmas or predicting the dependence of plasma emission on parameter variations. Although the development of such models for H2 started decades ago, major uncertainties still exist regarding the most important set of input parameters, namely the cross sections for electron-impact excitation. The deviations between cross sections from different datasets are particularly pronounced in the energy region close to the threshold energy, strongly increasing the uncertainty of CR models applied to low-temperature plasmas. This paper presents experimental validation of a set of newly calculated non ro-vibrationally resolved electron-impact cross sections calculated for the triplet system of H2 using the molecular convergent close-coupling method in the adiabatic-nuclei formulation. These cross sections are implemented into a CR model based on the flexible solver Yacora. A first comparison of CR calculations with the different datasets to experimentally-determined population densities is performed at a planar ICP discharge for varying pressure (between 1 and 10 Pa) and RF power (between 700 and 1100 W). For the experimentally-accessible electron temperature and density range (2.5–10 eV and 1.8–3.3 × 1016 m−3, respectively), very good agreement between the model and experiment is obtained using the new data set, in contrast to previously used cross sections.

Direct Ionization Cross Sections of Uranium Atom By Electron Impact

A theoretical model for electron impact ionization cross section has been found to be reliable for wide range of atoms is applied in this paper to the Uranium atom. A modified Kim binary encounter Bethe (BEB) method and modified Khare BEB method is employed for calculating electron impact ionization cross sections. The present results so obtained are compared with experimental as well as theoretical results known to the best of our knowledge.

Excitation and ionisation cross-sections in condensed-phase biomaterials by electrons down to very low energy: application to liquid water and genetic building blocks.

Electronic excitations and ionisations produced by electron impact are key processes in the radiation-induced damage mechanisms in materials of biological relevance, underlying important medical and technological applications, including radiotherapy, radiation protection in manned space missions and nanodevice fabrication techniques. However, experimentally measuring all the necessary electronic interaction cross-sections for every relevant material is an arduous task, so it is necessary having predictive models, sufficiently accurate yet easily implementable. In this work we present a model, based on the dielectric formalism, to provide reliable ionisation and excitation cross-sections for electron-impact on complex biomolecular media, considering their condensed-phase nature. We account for the indistinguishability and exchange between the primary beam and excited electrons, for the molecular electronic structure effects in the electron binding, as well as for low-energy corrections to the first Born approximation. The resulting approach yields total ionisation cross-sections, energy distributions of secondary electrons, and total electronic excitation cross-sections for condensed-phase biomaterials, once the electronic excitation spectrum is known, either from experiments or from a predictive model. The results of this methodology are compared with the available experimental data in water and DNA/RNA molecular building blocks, showing a very good agreement and a great predictive power in a wide range of electron incident energies, from the large values characteristic of electron beams down to excitation threshold. The proposed model constitutes a very useful procedure for computing the electronic interaction cross-sections for arbitrary biological materials in a wide range of electron incident energies.

Calculation of the single differential cross section for electron-impact ionization of atoms and molecules

A technique has been developed for calculating the electron-impact ionization single differential cross section directly from the integrated cross sections of positive-energy pseudostates occuring in close-coupling methods. Using the cross sections arising in the convergent close-coupling method, the approach is first tested against the existing benchmark theoretical and experimental data for electron scattering on hydrogen and helium. It is then applied to electron scattering on molecular hydrogen yielding excellent agreement with experimental data when normalized to the total ionization cross section.

Analysis of nighttime OH vibrational excited states production rate

Hydroxyl vibrational excited states OH∗ in the Earth’s ionosphere are investigated numerically in the presence of suprathermal electrons. The latter are included by using a Boltzmann equation numerical solver and a numerical cross-section for electron impact with OH. We have found that important changes in the peak altitude of OH∗ occur when geomagnetic activities vary. In addition, nighttime changes in solar activity and initial energy of precipitating electrons have minor effect on both peaks altitude and density of OH∗.

Slow magnetosonic wave absorption by pressure induced ionization–recombination dissipation

A new mechanisms for damping of slow magnetosonic waves (SMW) by pressure induced oscillations of the ionization degree is proposed. An explicit formula for the damping rate is quantitatively derived. Physical conditions where the new mechanism will dominate are briefly discussed. The ionization-recombination damping is frequency independent and has no hydrodynamic interpretation. Roughly speaking large area of partially ionized plasma are damper for basses of SMW while usual MHD mechanisms operate as a low pass filter. The derived damping rate is proportional to the square of the sine between the constant magnetic field and the wave-vector. Angular distribution of the spectral density of SMW and Alfv\'en waves (AW) created by turbulent regions and passing through large regions of partially ionized plasma is qualitatively considered. The calculated damping rate is expressed by the electron impact cross section of the hydrogen atom and in short all details of the proposed damping mechanisms are well studied.

The effect of H2O on the vibrational populations of CO2 in a CO2/H2O microwave plasma: a kinetic modelling investigation

Plasma has been studied for several years to convert CO2 into value-added products. If CO2 could be converted in the presence of H2O as a cheap H-source for making syngas and oxygenates, it would mimic natural photosynthesis. However, CO2/H2O plasmas have not yet been extensively studied, not by experiments, and certainly not computationally. Therefore, we present here a kinetic modelling study to obtain a greater understanding of the vibrational kinetics of a CO2/H2O microwave plasma. For this purpose, we first created an electron impact cross section set for H2O, using a swarm-derived method. We added the new cross section set and CO2/H2O-related chemistry to a pure CO2 model. While it was expected that H2O addition mainly causes quenching of the CO2 asymmetric mode vibrational levels due to the additional CO2/H2O vibrational-translational relaxation, our model shows that the modifications in the vibrational kinetics are mainly induced by the strong electron dissociative attachment to H2O molecules, causing a reduction in electron density, and the corresponding changes in the input of energy into the CO2 vibrational levels by electron impact processes.

X-ray spectra of the Fe-L complex

The Hitomi results for the Perseus cluster have shown that accurate atomic models are essential to the success of X-ray spectroscopic missions and just as important as the store of knowledge on instrumental calibration and astrophysical modeling. Preparing the models requires a multifaceted approach, including theoretical calculations, laboratory measurements, and calibration using real observations. In a previous paper, we presented a calculation of the electron impact cross sections on the transitions forming the Fe-L complex. In the present work, we systematically tested the calculation against cross-sections of ions measured in an electron beam ion trap experiment. A two-dimensional analysis in the electron beam energies and X-ray photon energies was utilized to disentangle radiative channels following dielectronic recombination, direct electron-impact excitation, and resonant excitation processes in the experimental data. The data calibrated through laboratory measurements were further fed into a global modeling of the Chandra grating spectrum of Capella. We investigated and compared the fit quality, as well as the sensitivity of the derived physical parameters to the underlying atomic data and the astrophysical plasma modeling. We further list the potential areas of disagreement between the observations and the present calculations, which, in turn, calls for renewed efforts with regard to theoretical calculations and targeted laboratory measurements.

Theoretical investigation of electron impact on formyl fluoride (HFCO)

In this paper, a descriptive and detailed theoretical analysis is carried out on electron scattering of formyl fluoride (HFCO) covering a wide impact energy range of 0.1 eV–5000 eV. Due to such a wide impact energy range, we were able to calculate the variety of interaction processes such as eigen phase study to infer about the resonances, vertical electron excitations, differential cross-sections, momentum transfer cross-sections, ionization cross-sections and total scattering cross-sections for electron impact on formyl fluoride. To compute these cross-sections for this energy range, we have used an amalgamation of two theoretical methodologies namely R-matrix via. Quantemol-N, valid for the low energy calculations from 0.1 eV to 20 eV and the Spherical Complex Optical Potential (SCOP) applicable from ionization threshold of the target to 5000 eV. There is a smooth cross over of total cross-sections observed between these two methods at the incident energy just below 20 eV. The present study becomes more important as it presents the electron impact cross-sections on HFCO for the first time and thus provides a reference data set for HFCO over an extensive impact energy range. The data set is important as Hydrofluorocarbons (HFCs) are one class of potential chlorofluorocarbons (CFC) substituents which mainly reacts with the OH radical in the atmosphere leading to the formation of the dominant intermediate product HFCO.

Effects of density on the oxygen ionization equilibrium in collisional plasmas

ABSTRACT The ion populations most frequently adopted for diagnostics in collisional plasmas are derived from the density independent coronal approximation. In higher density, lower temperature conditions, ionization rates are enhanced once metastable levels become populated, and recombination rates are suppressed if ions recombine into Rydberg levels. As a result, the formation temperatures of ions shift, altering the diagnostics of the plasma. To accurately model the effect of ionization from metastable levels, new electron impact ionization cross-sections have been calculated for oxygen, both for direct ionization and excitation–auto-ionization of the ground and metastable levels. The results have been incorporated into collisional radiative modelling to show how the ionization equilibrium of oxygen changes once metastable levels become populated. Suppression of dielectronic recombination has been estimated and also included in the modelling, demonstrating the shifts with density in comparison to the coronal approximation. The final results for the ionization equilibrium are used in differential emission measure modelling to predict line intensities for many lines emitted by O ii–O vi in the solar transition region. The predictions show improved agreement by 15–40 per cent for O ii, O vi, and the intercombination lines of O iii–O v, when compared to results from coronal approximation modelling. While there are still discrepancies with observations of these lines, this could, to a large part, be explained by variability in the observations.

Photoelectrons as a Tracer of Planetary Atmospheric Composition: Application to CO on Mars

AbstractPhotoelectrons are an extensively studied component of planetary ionospheres which have been frequently used as a diagnostic of ambient magnetic fields. We show in this study that they also provide information on atmospheric composition via the altitude variation of photoelectron intensity at specific energies. Such an idea is applied to Mars for which a large observational sample of photoelectrons is available from the Solar Wind Electron Analyzer measurements made by the Mars Atmosphere and Volatile Evolution (MAVEN). Our analysis reveals a strong decline in photoelectron intensity from 160 to 200 km, but this trend is restricted to a narrow energy range of 10–15 eV. By employing analytical yield spectrum calculations, we derive the model variations based on different choices of the background atmosphere. We find that the presence of CO is crucial for reproducing the observed variations. This allows atmospheric CO densities to be estimated from photoelectron measurements, which are a factor of 3–7 lower than the published densities based on the MAVEN Neutral Gas and Ion Mass Spectrometer measurements and in better agreement with existing model results. In general, the usefulness of photoelectron intensity at a specific energy as a tracer of atmospheric composition relies critically on the species‐dependent inelastic electron impact cross section at this energy.

Electron-impact cross-sections of atmospherically relevant amines from intermediate to 5000 eV energy range

The amines are major source of environment pollutants emitted in atmosphere from various anthropogenic sources. The non-thermal plasma (NTP)-based technology has proved successful in controlling the emitted amines reaching the atmosphere. The efficient NTP reactors rely on accurate electron–molecule collision data. The electron impact cross-sections are thus obtained for a few amines from ionisation threshold to 5000 eV using the single centre expansion (SCE) formalism. The molecular wave function of each target is obtained from the multicentre expansion of the Gaussian-type orbitals within a single determinant Hartree–Fock self-consistent field scheme. The expansion of wave function, density and potential is carried out at the centre of mass of the molecules. The interaction potential included to model the electron interaction in the target comprises static, correlation-polarisation and exchange types of potentials. The elastic cross-sections are obtained after solving the coupled scattering equations using Volterra integral form. The inelastic effects contributing to electron–molecule scattering are approximated by the ionisation cross-sections. The total cross-sections obtained after summing the elastic and ionisation cross-sections are in good agreement with the available data. We have also tried to explain the effect of polarisation potential on scattering cross-sections. A semiempirical formula based on the spatial extent of the molecule is proposed to estimate the cross-sections for the homologous series of amine molecules.