Total Excitation Cross Sections Research Articles

Calculations of positron scattering from small molecules

Recently, convergent close-coupling calculations have been completed for positron scattering from the carbon and oxygen atomic targets. These, together with previously completed calculations for atomic hydrogen, are utilized to perform positron scattering calculations for molecular hydrogen (H2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {H}_2$$\\end{document}), molecular oxygen (O2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {O}_2$$\\end{document}), diatomic carbon (C2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {C}_2$$\\end{document}), carbon monoxide (CO), carbon dioxide (CO2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {CO}_2$$\\end{document}), ozone (O3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {O}_3$$\\end{document}), water (H2O\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {H}_2\\hbox {O}$$\\end{document}), and methane (CH4\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\hbox {CH}_4$$\\end{document}) through a modified independent atom approach. For these molecules, positronium-formation, direct ionization, electron-loss, elastic, total electronic excitation, total inelastic, and total cross sections are obtained for energies between 0.1 and 5000 eV. There is, in general, good agreement between the current results and past experiments for most transitions, particularly at high energies where this approach is expected to be most accurate.Graphic

Analysis of the cross section and characteristic of fluorescence emission of the He atom confined in the plasma conditions

The structural properties, angle-integrated total and magnetic sublevel excitation cross sections, alignments of the residual ion, and polarization properties of the line emission are investigated for the plasma-embedded atomic system. This is done using the developed distorted wave approach which solves the Dirac equation by employing a modified Debye potential to represent the screened interaction in plasmas. The effective Hamiltonian is employed to obtain the level energies as functions of the screening parameters. The Coulomb interaction is replaced by shielded Coulomb interaction during the collision dynamics. As an illustrating example, detailed calculations are carried out for the 1s2→1s2p3,1P1 excitations of the plasma-embedded He atom by electron impact. Our results suggest that the energy levels are pushed towards continuum as the effect of (plasma) confinement increases. The cross sections decreases monotonically if the screening effect is enhanced continuously, while the alignments of the residual He+ ion and polarizations of the fluorescence emission change slightly for the corresponding screening parameters. A comparison of our results in the absence of the screening potential with other available experimental and theoretical data shows satisfactory agreement. The present study not only provides a helpful approach to study the influence of plasma effect, but also has important applications in laser produced plasmas, stellar atmospheres, fusion research plasmas and so on.

The Excitation of the 3D and 4D States of Atomic Hydrogen by Electron Impact

The excitation cross-sections of the 3D and 4D states of atomic hydrogen at low incident energies (from 0.90 to 5.00 Ry) were calculated using the variational polarized orbital method, which is also called the hybrid theory. Up to 12 partial waves (L = 2 to 13) were used to obtain converged cross-sections at high energies. The importance of the long-range forces near the threshold region and the behavior of the cross-sections in that region are indicated. The S, P, and D cross-sections are needed if the total excitation cross-sections are measured in addition to the elastic cross-sections. These cross-sections are also useful if the cascade from the D to the P to the S states is considered in the diagnostics of solar and astrophysical observations.

Convergent close-coupling calculations of positron scattering from atomic carbon

We report on the extension of the single-center convergent close-coupling method to positron scattering on multielectron atoms, along with the development of a technique for separating the direct ionization and positronium-formation channels in single-center calculations. We have performed calculations of positron scattering on carbon and present total elastic, momentum transfer, excitation, direct ionization, total ionization, total inelastic, positronium-formation, stopping power, and total cross sections from threshold to 5000 eV. We also present oscillator strengths, the scattering length, the hidden Ramsauer-Townsend minimum, the energy of the positron-carbon virtual state, and the mean excitation energy. Agreement with electron-scattering experiment and positron-scattering theory for several cross sections has been demonstrated for high energies. However, discrepancies exist between different theoretical methods at lower energies and for the ionization and positronium-formation processes.

Electron interactions with analogous of DNA/RNA nucleobases: 3-hydroxytetrahydroFuran and α-Tetrahydrofurfuryl alcohol

We report quantitative results for electron induced molecular processes for DNA/RNA nucleobase analogous bio-molecules, 3-hydroxytetrahydrofuran (C4H8O2, 3hTHF) and α-tetrahydrofurfuryl alcohol (C5H10O2, THFA), which are similar in structure and functional group to backbone of DNA and RNA. We have employed spherical complex optical potential (SCOP) formalism to compute total elastic (Qel) and inelastic (Qinel) cross sections and used Complex Scattering Potential-ionization contribution (CSP-ic) method to obtain total ionization (Qion) and summed total excitation (∑Qexc) cross sections. These results obtained for a wide energy range (10 eV - 5 keV) may serve as the input parameters in predicting damage in biomolecular systems induced by secondary electrons caused by primary ionization radiation. We have observed a correlation between maximum Qion and molecular polarizability volume (α), ionisation energy (IE) and number of valence electrons (Nv). This work includes an attempt to estimate ∑Qexc for these molecules in normal state.

Electron impact inelastic molecular processes for deuterated compounds

In this theoretical work, electron induced inelastic molecular processes for deuterated molecules HD, D2, D2O, SiDx (x = 1–3), NDx (x = 1–3) and CDx (x = 2–4) are studied and quantified from ionisation threshold to 5 keV. The total inelastic cross section (Qinel) are computed using Spherical Complex Optical Potential (SCOP) and total ionisation cross sections (Qion) and total electronic excitation cross sections (∑Qexc) have been evaluated through the Complex Spherical Potential – ionisation contribution (CSP-ic) method. Electron driven inelastic effects for deuterated hydrogen (HD) are investigated for the first time in this study. This is the first report of total Qion for HD, SiDx (x = 1–3), NDx (x = 1,2), CDx (x = 2,3) molecules and maiden study of Qinel and ∑Qexc for all these molecules. Further, isotope effect is studied for these targets and using various correlation analyses, dipole polarizability for ND and SiDx (x = 1–3) molecules are predicted.

English

Excitation of 3G° levels of the nickel atom was investigated using the method of extended atomic and electron crossing beams with registration of the optical signal of excited atoms. At an energy of exciting electrons of 50 eV, 14 excitation cross sections were measured. In the electron energy range of 0 to 200 eV, optical excitation functions (OEF) were recorded for transitions from five levels. Based on the results obtained, the total level excitation cross sections are calculated. All these results are received for the first time. Key words: Nickel atom, cross section, optical excitation function (OEF), energy level, spectral line.

Positron scattering from pyrazine

The present paper provides a joint experimental and theoretical study of positron scattering from pyrazine. Experimental data were measured employing a low-energy positron beamline, and covered an energy range from 1 to 79 eV. Cross sections were measured for total scattering, total elastic scattering, positronium formation, direct ionization, and the sum of the total discrete electronic-state excitation processes. In addition, measured total electronic excitation cross sections for the individual ${1}^{1}{B}_{3u},$ ${1}^{1}{B}_{2u},$ ${1}^{1}{B}_{1u},$ and ${2}^{1}{B}_{2u}$ states are also reported. Finally, experimental elastic differential cross sections (DCSs), here in the energy range $1--20$ eV, are also presented. Results from two theoretical approaches are given: the R-matrix approach and the independent atom method with screening corrected additivity rule and interference terms. Where a comparison between our experimental and theoretical results could be made, the level of accord varied from being quite good to marginal depending on the actual scattering process under consideration. Last, a comparison between the present elastic DCS results and those from our earlier study on positron scattering from pyrimidine [Palihawadana et al., Phys. Rev. A 88, 012717 (2013)] showed a somewhat unexpected level of agreement.

Charge transfer and excitation processes in low energy collisions of He+ ions with Li atoms

Electron capture between solar wind ions and neutral species has contributed to the understanding of X-ray production from solar system bodies. The charge transfer and excitation processes in solar wind ions of He+(1s) colliding with Li(1s 22s) atoms are studied by utilizing the full quantum-mechanical molecular-orbital close-coupling (QMOCC) method with impact energies of 0.003–2 keV amu−1. Comparisons of cross sections from single- and multi-configurational calculations for a self-consistent field (SCF and MCSCF) process are carried out. Results show that the dominant reaction channels are He(1s2l 1,3L) + Li+(1s 2 1S). Good consistency is found among present total and state-selective charge transfer and excitation cross sections with other theoretical and experimental data in the same energy region. Due to the differences between coupling matrix elements in high-energy states, the charge transfer cross sections calculated from SCF and MCSCF split slightly as E > 0.4 keV amu−1. Weak Stueckelberg oscillations for charge transfer appear in the present work. In addition, the differences of cross sections for electron excitation to Li(1s 22p) in the singlet/triplet molecular states with He+(1s) are much smaller than those of charge transfer processes because of the similar energy gaps from Li(1s 22p) to the ground state in singlet/triplet states in the large R region.

Electron induced elastic and inelastic processes for perfluoroketone (PFK) molecules

We report the results of calculations of elastic and inelastic (ionization and excitation) cross sections for electron scattering from perfluoroketone molecules, C x F2x O (x = 1–6) over a wide energy range, from ∼the ionization potential to 5 keV. These molecules have been determined to have extremely low global warming potentials and therefore may have applications in next generation gas discharges and plasma reactors. The results are derived using the complex scattering potential-ionization contribution method to investigate ionization cross sections Q ION and are found to be in good agreement with the available data. The spherical complex optical potential formalism is used to evaluate elastic (Q EL), inelastic (Q INEL) and total cross sections (Q T). This study is a maiden effort to report summed total excitation cross section (∑Q EXC), Q ION and Q INEL for CF2O and C2F4O, and Q EL and Q T for C x F2x O (x = 1–6). The study includes various correlation analyses and a prediction of the dipole polarizability.

Excitation cross sections in a collision between two ground-state hydrogen atoms

We present excitation cross sections in collision between two ground state hydrogen atoms using a four-body classical trajectory Monte Carlo model. Calculations were performed for impact energies in the range between 1.0 keV and 100 keV where the cross sections are highly relevant to the interest of the fusion research. Beside the total excitation cross sections for target and projectile we also present partial excitation cross sections into the 2s and 2p states of the target. The partial excitation cross sections are compared with the previously obtained theoretical and experimental results.

Interaction of Be4+ and Ground State Hydrogen Atom—Classical Treatment of the Collision

The interaction between Be4+ and hydrogen atom is studied using the three-body classical trajectory Monte Carlo method (CTMC) and the quasiclassical trajectory Monte Carlo method of Kirschbaum and Wilets (QTMC-KW). We present total cross sections for target ionization, target excitation, and charge exchange to the projectile bound states. Calculations are carried out in the projectile energy range between 10 and 1000 keV/au, relevant to the interest of fusion research when the target hydrogen atom is in the ground state. Our results are compared with previous theoretical results. We found that the classical treatment describes reasonably well the cross sections for various final channels. Moreover, we show that the calculations by the QTMC-KW model significantly improve the obtained cross sections.

K-shell electron-impact excitation and polarization of x-ray emission of Fe20+ ion

SynopsisThe total and magnetic cross sections for K-shell electron impact excitation (EIE) of Fe20+ have been calculated using the fully relativistic distorted-wave approach. Degrees of linear polarization of x-ray emission following EIE are presented for several stronger 2p→1s transitions, and compared with the values for the same transition lines but from dielectronic recombination process of Fe21+ in the ground state, obvious differences are found that can be employed in diagnosing the formation mechanism of corresponding x-ray lines.

Polarization of fluorescence radiation following electron impact excitation of ions immersed in strongly coupled plasmas

An investigation on the electron impact excitation and subsequent radiative decay process of He-like Fe24+ ions immersed in strongly coupled plasmas is made, using the 1s2p P3,11 → 1s2 S10 characteristic lines as an example. The shielded nuclear potential of the uniform electron gas model (UEGM) type experienced by the electron is parameterized by the ion-sphere radius. For the target structure, accuracies of wave functions are justified by evaluating the energies of the relevant states. To serve as an independent check of the results, self-consistent calculations are carried out using the multiconfiguration Dirac-Fock relativistic configuration interaction method incorporating the same potential. For the impact excitation process, the UEGM potential is also employed to screen the projectile electron from the nucleus and target electrons. A distorted-wave method in the framework of the relativistic theory is developed to include the effect of plasma background, in which the continuum wave function of the projectile electron is evaluated by solving numerically the modified Dirac equations. The plasma effects on the total excitation cross section, magnetic sublevel cross section, and linear polarization of fluorescence radiation are investigated. The comparison with the available theoretical and experimental results is satisfactory.

Electron capture, excitation and ionization in H+–Be+ collisions

State-selective single electron capture, excitation and ionization processes in proton collisions with Be+(1s22s) and Be+(1s22p) ions are studied by using the quantum-mechanical molecular orbital (QMOCC) and the two-center atomic orbital close-coupling (TC-AOCC) methods in the combined energy range 0.01–300 keV/u. In the QMOCC calculations six 2Σ+ and three 2Π states are included in the expansion basis. The TC-AOCC expansion basis includes 35 discrete states centered on the proton and 182 states on Be2+ out of which 88 are continuum pseudostates. The atomic states on Be2+ are generated by a one-electron model potential for the electron interaction with Be2+ ion core that reproduces the exact energies with accuracy better than 0.5%. Total and state-selective electron capture and excitation cross sections to final states with n ≤ 4 are presented. The sum of the cross sections for the transitions to continuum pseudostates (ionization cross section) is also reported for both initial target states. The physical mechanisms involved in the dynamics of considered processes are discussed in detail at both low and high energies. The reported cross sections should be useful in the kinetic modeling and diagnostics of edge plasmas in present magnetic fusion experiments, as well as in the current ITER design, in which beryllium is used as first wall material.

Electron capture and excitation in H+ − Li+ collisions

The processes of state-selective electron capture and excitation in proton collisions with Li+(1s2 1S) and Li+(1s2s 1,3S) ions are studied by using the two-center atomic orbital close-coupling (TC-AOCC) methods in the energy range 2.5–800 keV. The interaction of the active electron with the Li2+ ion core is represented by model potentials that reproduce the energies of excited singlet and triplet states of Li+ with accuracy better than 2%. The expansion basis in the coupled state calculations includes all states with n ≤ 8 (in total 120 states) centered on each of the colliding centers. Total and state-selective electron capture and excitation cross sections to final states with n ≤ 4 are presented. The physical mechanisms involved in the dynamics of considered processes are discussed in detail. The reported cross sections should be useful in the kinetic modeling and diagnostics of fusion reactor plasmas with liquid lithium divertors.

Total Absorption Cross Section for UV Excitation of Sulfur Monoxide.

In a new study on sulfur monoxide, new ab initio potential energy curves are developed for excited states A3Π, B3Σ-, C3Π, and C'3Π and for three states that are unassigned in the literature, which we numerically name 33Σ-, 43Π, and 53Π. All these excited states have allowed transitions from ground state, X3Σ-. The ab initio calculations were performed using the MRCI-F12+Q/aug-cc-pV(5+d)Z level of theory implemented in MOLPRO2015. On the basis of close-coupling R-matrix theory, fine structure absorption cross sections of isotopically substituted sulfur monoxide are calculated for wavelengths of 190-300 nm. The spectra are shown at the highest possible resolution (FWHH ≈ 0.15-0.18 cm-1) for reference and future studies. The effects of self-shielding and possible mutual shielding are discussed.

Electron interactions with astro chemical compounds

In present work electron induced processes with important astro-compounds found in the tholins of Titan are investigated. We report calculated total elastic cross sections Qel, total inelastic cross sections Qinel, total ionization cross sections Qion, total excitation cross sections ∑Qexc and total cross sections QT for hydrogen cyanide (HCN), cyanoacetylene (HCCCN), vinyl cyanide (CH2CHCN), methanimine (CH2NH) and ethanimine (CH3CHNH) on electron impact for energies from ionization threshold to 5 keV. We have employed the Spherical Complex Optical Potential (SCOP) formalism to investigate elastic as well as inelastic processes and used Complex Scattering Potential – ionization contribution (CSP-ic) method to derive ionization cross sections. In absence of any theoretical or experimental data of ionization cross sections except for HCN and HCCCN, we have computed Qion using the Binary- Encounter- Bethe (BEB) method for all these molecules and have found reasonable agreement. This is the maiden attempt to report various total cross sections for all these astro-molecules except HCN and HCCCN.

Electron and Positron Impact Ionization Cross Sections of O and Hg Atoms

Total ionization crosssections for O and Hg atoms have been calculated for electron and positron impact scattering. A quantum mechanical formulation originating from complex atomic spherical potential is adapted in the scattering calculations on two targets. Then we obtain total ionization cross sections and cumulative total excitation cross sections by applying variant CSP-ic method. Our calculated results are presented for impact energy starting from the first threshold of ionization and going up to about 2000 electron volts (eV). For positron-atom scattering, our total inelastic cross sections include positronium formation together with ionization and excitation channels. Because of Ps formation channel it is difficult to separate our ionization cross sections from the total inelastic cross sections. An approximate method has been applied in this regard. Electron impact studies on Hg atom are scarce in literature. Similarly positron impact ionization in relation to electronic ionization discussed presently has not been highlighted much.

Electron and positron scattering cross sections for propene

The molecular R-matrix calculation is performed for the low energy electron scattering from propene to obtain total and rotational excitation cross sections. The rotational excitation cross sections are calculated for the transitions involving levels from j = 0 to j = 5. These cross sections are used for calculating their corresponding rate coefficients for the temperature range 30–5000 K. The rate coefficients at such temperatures have relevance in the interstellar medium as well as in the plasma environment. The spherical complex optical potential and complex scattering potential- ionization contribution methods are used for high energy calculation of the total cross section (Qtot) and ionization cross section (Qion), respectively, for both electron and positron scattering. The present Qtot results give reasonable agreement with previous experimental data. We have detected a π* shape resonance for electron scattering at 3.71 eV due to 2A″ scattering state, which is in accordance with the experimental results. However, no such resonance is observed in the positron scattering calculations. The cross sections reported here show good comparison with existing data, validating present calculations and its reliability.