Calculation Of Ionization Cross Sections Research Articles

Aromatic hydrocarbons as Molecular Propellants for Electric Propulsion Thrusters

The aromatic hydrocarbons (AHs) fluorobenzene, naphthalene, and 1-fluoronaphthalene are introduced as promising alternatives to xenon as propellant for in-space electric propulsion (EP). These storable molecules have similar mass, lower cost, and lower ionization energies compared to xenon, as well as the critical advantage of low post-ionization fragmentation compared to other molecular propellant candidates. The ionization characteristics of AHs are compared with those of xenon and the diamondoid adamantane, previously evaluated as a molecular propellant for EP. Quantum chemical calculations and BEB theory together with 25 eV electron ionization mass spectrometry (EI-MS) measurements have been used to predict the fragmentation of the AHs and adamantane when ionized in a plasma with an electron temperature of 7 eV (a typical electron temperature in EP plasmas). A high fraction (81 − 86%) of the detected AH ions originate from intact molecules, compared to 34% for adamantane, indicating extraordinarily low fragmentation for the selected AHs. The ionization potential of the AHs is similar to that of adamantane but lower compared to xenon (8.14–9.2 eV for the AHs, 9.25 for adamantane and 12.13 eV for xenon). BEB calculations have also been used to predict total ionization cross sections. The calculated ionization cross section of the AHs is comparable to that of adamantane but 3–5 times higher than that of xenon, which together with the low ionization potential can contribute to more efficient ionization. The AHs may have the potential to perform better than xenon, despite the absence of fragmentation in xenon.

Charge-state evolution from W5+ to W7+ at energies below the ionization potentials

Experiments on an electron-beam ion trap (EBIT) and calculations using flexible atomic code (FAC) are carried out to study the charge-state evolution from ${\mathrm{W}}^{5+}$ to ${\mathrm{W}}^{7+}$. The ${\mathrm{W}}^{7+}$ line at 574.47 nm is observed with an electron-beam energy of about 48 eV, which is far below the ionization potentials of ${\mathrm{W}}^{5+}$ (65 eV) and ${\mathrm{W}}^{6+}$ (122 eV). Multicharge-state collisional-radiative (CR) calculations for ${\mathrm{W}}^{5+}, {\mathrm{W}}^{6+}$, and ${\mathrm{W}}^{7+}$ are performed with level-to-level processes with configuration interaction (CI), including direct ionization, collision excitation, radiative recombination, charge exchange, radiative transition, and autoionization. The CI strongly influences the calculated ionization cross sections for metastable levels. The CR-simulated spectra agree well with the experiments, and the calculated effective ionization cross section for ${\mathrm{W}}^{6+}$ has the same trend as the available experimental data [M. Stenke et al., J. Phys. B: At. Mol. Opt. Phys. 28, 2711 (1995)]. The metastable levels (\ensuremath{\sim}40 eV for ${\mathrm{W}}^{5+}$; \ensuremath{\sim}40 and \ensuremath{\sim}85 eV for ${\mathrm{W}}^{6+}$) significantly contribute to the ionization through excitation-autoionization at rather low energies (50 eV) in the EBIT plasma. These metastable levels could have a considerable influence on the charge-state evolution of tungsten ions in edge fusion plasma.

Study of Electron Impact Ionization Cross-Sections of Nitrogen Molecule by Inelastic Collision

In this article, the calculation of ionization cross-section of N2 molecule by electron impact is reviewed. The work on ionization cross-sections from threshold to high (10 KeV) incident electron energies are calculated and their applications are discussed. To find exact values of total ionization cross-sections for the N2 molecule, a semiempirical approach of Jain and Khare (1976) is adopted which is a modification to eliminate the error factors, and then it is compared with the existing experimental and/or theoretical data which gives satisfactory results

Study of the physical step interaction of the proton with DNA molecules using analytical approach and Monte-Carlo simulation

Ionization cross-section calculations are analytically presented for proton collisions from DNA nucleobases guanine, adenine, thymine, and cytosine based upon the semi-empirical Rudd model. New parameterizations are used to calculate the excitation and change–charge cross-sections from liquid water. We explored the differential electron cross-sections of biological atom shells to calculate the total stopping cross section for protons with DNA bases. We, therefore, studied the Bragg peak position for protons in a wide range from 100 eV to 20 MeV of impact energy. The obtained results showed a reasonable agreement with available experimental data and calculations from Monte-Carlo (MC) method, GEANT4, and SRIM toolkits.

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.

Electron capture and ionization cross-section calculations for proton collisions with methane and the DNA and RNA nucleobases

Net ionization and net capture cross-section calculations are presented for proton collisions from methane molecules and the DNA/RNA nucleobases adenine, cytosine, guanine, thymine, and uracil. We use the recently introduced independent-atom-model pixel counting method to calculate these cross sections in the 10 keV to 10 MeV impact energy range and compare them with results obtained from the simpler additivity rule, a previously used complete-neglect-of-differential-overlap method, and with experimental data and previous calculations where available. It is found that all theoretical results agree reasonably well at high energies, but deviate significantly in the low-to-intermediate energy range. In particular, the pixel counting method which takes the geometrical overlap of atomic cross sections into account is the only calculation that is able to describe the measurements for capture in proton-methane collisions down to 10 keV impact energy. For the nucleobases it also yields a significantly smaller cross section in this region than the other models. New measurements are urgently required to test this prediction.

The Electron Impact Ionization Cross Sections of Methanol, Ethanol and 1-Propanol

In the present investigation, the plane-wave Born approximation was employed to calculate the total ionization cross sections by electron impact of methanol, ethanol and 1-propanol from the threshold of ionization to 10 MeV. This method requires continuum generalized oscillator strengths (CGOSs). The two different semi-phenomenological expressions of CGOS, given by Mayol and Salvat and Weizsacker and Williams, along with approximated form of the continuum optical oscillator strength (COOS) by Khare et al. were used. Furthermore, the average of the above two CGOSs was also used. The calculated ionization cross sections were compared to the available previous theoretical results and experimental data. Out of three CGOSs, the present results with the average CGOS were found in good agreement with the available experimental results for all the considered molecules. Collision parameters CRP were also calculated from 0.1 to 100 MeV and the calculations were found to be in excellent agreement with the experimental results of Reike and Prepejchal.

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.

Excitation of Autoionization States and Electronic Stopping Powers at Collisions of Slow Ions with a Solid

It has been shown that the excitation of autoionization states at collisions of keV ions with a solid is decisive for inelastic energy loss and, correspondingly, the electronic stopping power dE/dx. It has been proposed to estimate the electronic stopping power dE/dx using the relation of cross sections for the excitation of autoionization states to ionization cross sections. When ionization cross sections are unknown, scaling is used to calculate ionization cross sections at the excitation of the L and M shells. A threshold dependence of the electronic stopping power dE/dx on the energy of bombarding ions has been predicted.

Electron-impact single ionizaiton for W4+ and W5+**Project supported by the National Natural Science Foundation of China (Grant Nos. 2017YFA0402300, 11464042, 11564036, 11774292, and 11464040) and the Basic Scientific Research Foundation for Institution of Higher Learning of Gansu Province, China.

Electron-impact single ionization cross sections for Wq+ (q = 4–5) were calculated using the flexible atomic code (FAC) in the level-to-level distorted-wave method, considering the explicit branching ratio. The calculated cross sections are compared with the available theoretical and experiment results in detail. In the case of the contribution from the same channles as the available theoretical results, all of the calculated ionization cross sections agree with the experimental measured cross sections. But the present calculated results are larger than the experimental measurement when all channels contributions are included. Some important channels excitation autoionization (EA) contributions, such as the excitation to higher higher subshell from 4f and 5[s,p], were not included into the available theoretical calculation. In general, the distorted-wave (DW) results are overestimated.

Single and double over-barrier ionization of He, He+ and Ne system by positron impact

The classical over-barrier ionization model (COBI) method and trajectory calculations were utilized to simulate the ionization of He+ impacted by a positron. The calculated ionization cross sections of He+ agree with other theoretical data. Additionally, we found that the double ionization of He has a definite association with the positron-He+ impact. This result can explain why doubly ionized He seemed to be positron-scattered by the rest of the He+ in our previous study. The COBI model was also extended to study the double ionization caused by positron-Ne impacts. Our theoretical results agree with the experimental data.

Electron impact scattering and calculated ionization cross sections for SFx (x = 1–5) radicals

The spherical complex optical potential (SCOP) formalism is employed to solve the e−-SFx (x=1–5) scattering system. In this article, the total cross sections by electron impact from 50 to 5000eV are calculated. The complex scattering potential ionization contribution (CSP-ic) method is used to compute the electron-induced total ionization cross sections from the inelastic cross section in the energy range from ionization threshold to 5000eV. For most of the reported radicals, the magnitude and shape of cross section compares well with previous measurements and calculations, wherever available. However, for many targets results are predicted for the first time in this work. From the electron-impact scattering cross sections for the SFx (x=1–5) radicals, we also estimate the gas-kinetic radius and the van der Waals coefficient.

X-ray emission from thin plasmas

Every observation of astrophysical objects involving a spectrum requires atomic data for the interpretation of line fluxes, line ratios and ionization state of the emitting plasma. One of the processes which determines it is collisional ionization. In this study an update of the direct ionization (DI) and excitation-autoionization (EA) processes is discussed for the H to Zn-like isoelectronic sequences. In the last years new laboratory measurements and theoretical calculations of ionization cross sections have become available. We provide an extension and update of previous published reviews in the literature. We include the most recent experimental measurements and fit the cross sections of all individual shells of all ions from H to Zn. These data are described using an extension of Younger's and Mewe's formula, suitable for integration over a Maxwellian velocity distribution to derive the subshell ionization rate coefficients. These ionization rate coefficients are incorporated in the high-resolution plasma code and spectral fitting tool SPEX V3.0.

X-ray and kinetic electron emission by keV proton impacting on fusion-relevant tungsten

We have measured the electron and X-ray emission of tungsten bombarded with proton in the energy range of 50–250keV. It is found that the total electron yield mainly consists of kinetic electron emission, which results from the excited valence electrons, and presents a similar trend to the curve of stopping power. The ratio between electron yield and electronic energy loss is not constant, but decreases with increasing incident energy. The behavior is interpreted by an expected competitive mechanism of ionization between different shell electrons of target atom. The explanation is verified by the experimental increase of M-shell X-ray emission yield, and the measured and calculated ionization cross section of core and valence electrons.

DiracR-matrix calculations of photoionization cross-sections of Ni xiii

In this paper, we report total photoionization cross-sections of Ni xiii in the ground state (3P2) and four excited states (3P1,0, 1D2, 1S0) for the first time over the photon energy range 380–480 eV. The target wavefunctions are constructed with fully relativistic atomic structure GRASP code. Our calculated energy levels and oscillator strengths of core ion Ni xiv agree well with available experimental and theoretical results. The ionization threshold value of ground state of Ni xiii is found to be more closer to the experimental ionization energy and improved over the previous calculations. The photoionization cross-sections are calculated using the fully relativistic darc code with an appropriate energy step of 0.01 eV to delineate the resonance structures. The calculated ionization cross-sections are important for the modelling of features of photoionized plasmas and for stellar opacities.

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.

Theoretical triple-differential cross sections of a methane molecule by a proper-average method

For the last few years, our group has calculated cross sections for electron-impact ionization of molecules using the molecular three-body distorted-wave approximation coupled with the orientation-averaged molecular orbital (OAMO) approximation. This approximation was very successful for calculating ionization cross sections for hydrogen molecules and to a lesser extent nitrogen molecules. Recently we used the approximation to calculate single ionization cross sections for the $1{t}_{2}$ state of methane (CH${}_{4}$) and we found major discrepancies with the experimental data. Here we investigate the validity of the OAMO approximation by calculating cross sections that have been properly averaged over all molecular orientations. These calculations with proper averages are in much better agreement with experiment than the OAMO calculations.

Modelling of turbulent impurity transport in fusion edge plasmas using measured and calculated ionization cross sections

Turbulent transport of trace impurities in the edge and scrape-off-layer of tokamak fusion plasmas is modelled by three dimensional electromagnetic gyrofluid computations including evolution of plasma profile gradients. The source function of impurity ions is dynamically computed from pre-determined measured and calculated electron impact ionization cross section data. The simulations describe the generation and further passive turbulent E×B advection of the impurities by intermittent fluctuations and coherent filamentary structures (blobs) across the scrape-off-layer.

Cross sections for ionization of uracil by MeV-energy-proton impact

Absolute double differential cross sections (DDCS) have been measured for the ionization of gas-phase uracil (C${}_{4}$H${}_{4}$N${}_{2}$O${}_{2}$) by 0.5, 1.0, and 2.0 MeV proton impacts. Measurements were performed by secondary electron spectroscopy using a 45${}^{\ensuremath{\circ}}$ parallel plate electrostatic spectrometer over an energy range of 1.0--1000 eV at emission angles from 15${}^{\ensuremath{\circ}}$ to 165${}^{\ensuremath{\circ}}$. Theoretical calculation of ionization cross sections has also been carried out by using the first-order Born approximation with correct boundary conditions. Single differential and total ionization cross sections deduced from DDCS revealed fairly good agreement between experimental and theoretical values at all the emission angles investigated.

Electron impact ionization cross sections of the CO2 clusters

The modified Jain–Khare semi-empirical formalism for the evaluation of differential and integral electron impact ionization cross sections for molecules has been extended to the evaluation of cross sections for the electron ionization of CO2 clusters: (CO2)240 and (CO2)1700. The energy dependent differential cross sections are evaluated at the incident electron energies of 50, 100 and 200eV. The integral total ionization cross sections have been calculated in the energy range varying from ionization thresholds to 1000eV which revealed a good agreement with the available experimental and the theoretical data. The ionization rate coefficients have also been evaluated using the presently calculated ionization cross sections and Maxwell–Boltzmann energy distributions.