Electron-impact Single Ionization Research Articles

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)].

STORAGE RING CROSS SECTION MEASUREMENTS FOR ELECTRON IMPACT IONIZATION OF Fe11+FORMING Fe12+AND Fe13+

We report ionization cross section measurements for electron impact single ionization (EISI) of Fe^11+$ forming Fe^12+ and electron impact double ionization (EIDI) of Fe^11+ forming Fe^13+. The measurements cover the center-of-mass energy range from approximately 230 eV to 2300 eV. The experiment was performed using the heavy ion storage ring TSR located at the Max-Planck-Institut fur Kernphysik in Heidelberg, Germany. The storage ring approach allows nearly all metastable levels to relax to the ground state before data collection begins. We find that the cross section for single ionization is 30% smaller than was previously measured in a single pass experiment using an ion beam with an unknown metastable fraction. We also find some significant differences between our experimental cross section for single ionization and recent distorted wave (DW) calculations. The DW Maxwellian EISI rate coefficient for Fe^11+ forming Fe^12+ may be underestimated by as much as 25% at temperatures for which Fe^11+ is abundant in collisional ionization equilibrium. This is likely due to the absence of 3s excitation-autoionization (EA) in the calculations. However, a precise measurement of the cross section due to this EA channel was not possible because this process is not distinguishable experimentally from electron impact excitation of an n=3 electron to levels of n > 44 followed by field ionization in the charge state analyzer after the interaction region. Our experimental results also indicate that the double ionization cross section is dominated by the indirect process in which direct single ionization of an inner shell 2l electron is followed by autoionization resulting in a net double ionization.

Electron-impact ionization and dissociation of C2D+

Absolute cross sections for electron-impact single ionization, dissociative excitation and dissociative ionization of the ethynyl radical ion (C(2)D(+)) have been measured for electron energies ranging from the corresponding reaction thresholds to 2.5 keV. The animated crossed electron-ion beam experiment; is used and results have been obtained for the production of C(2)D(2+), C(2+), C(2)(+), CD(+), C(+) and D(+). The maximum of the cross section for single ionization is found to be (2.01 +/- 0.02) x 10(-17) cm(2), at the incident electron energy of 105 eV. Absolute total cross sections for the various singly charged fragments production are observed to decrease by a factor of almost three, from the largest cross-section measured for C(+), over C(2)(+) and CD(+) down to that of D(+) The maxima of the cross sections are obtained to be (14.5 +/- 0.5) x 10(-17) cm(2) for C(2)(+) (12.1 +/- 0.1) x 10(-17) cm(2) for CD(+) (27.7 +/- 0.2) x 10(-17) cm(2) for C(+) and (11.1 +/- 3.8) x 10(-17) cm(2) fo D(+). The smallest cross section is measured to be (1.50 +/- 0.04) x 10(-18) cm(2) for the production of the doubly charged ion C(2+). Individual contributions for dissociative excitation and dissociative ionization are determined for each singly-charged product. The cross sections are presented in closed analytic forms convenient for implementation in plasma simulation codes. Kinetic energy release distributions of dissociation fragments are seen to extend from 0 to 6 eV for the heaviest; fragment C(2)(+), up to 11.0 eV for CD(+) 14.2 eV for C(+) and 11.2 eV for D(+) products.

Electron impact multiple ionization of argon ions

Absolute cross sections for electron impact single and multiple ionization of argon ions leading to the formation of Ar q+ (q = 2–8) are reported. The animated crossed beam method is applied in the energy range extending from the respective thresholds up to 2.5 keV. The maximum cross sections for the multiply charged products Ar q+ (q = 2–8) are measured to be (12.6 ± 0.6) × 10−17 cm2 for Ar2+, (4.96 ± 0.21) × 10−18 cm2 for Ar3+, (5.50 ± 0.23) × 10−19 cm2 for Ar4+, (8.10 ± 0.33) × 10−20 cm2 for Ar5+, (8.2 ± 0.7) × 10−21 cm2 for Ar6+, (8.9 ± 0.8) × 10−22 cm2 for Ar7+ and (5.5 ± 1.6) × 10−23 cm2 for Ar8+. The corresponding threshold energies are determined to be (27.4 ± 0.5) eV, (68.7 ± 0.5) eV, (135 ± 2) eV, (250 ± 10) eV, (310 ± 10) eV, (480 ± 20) eV and (640 ± 40) eV for the production of Ar q+ (q = 2–8), respectively. The direct process is seen to dominate for q = 2–3, while the indirect processes are dominant for charge states 4–8.

An extended empirical formula for inner-shell ionization of atoms

An extension of the analytical model of Campos et al (2007 J. Phys. B: At. Mol. Opt. Phys. 40 3835) is proposed to evaluate electron impact single inner-shell ionization cross sections up to the M-shell. The new model includes ionic and relativistic factors in its structure and describes neatly the K-shell ionization cross section data up to 2 GeV, and L- and M-shell ionization data up to 300 MeV. Comparison is also made with other theoretical calculations.

The Distorted-Wave Born Approach for Calculating Electron-Impact Ionization of Molecules

The distorted-wave Born approximation (DWBA) has been one of the most successful theoretical approaches for treating electron collisions with complicated atoms, and recently the DWBA has been successfully extended to treat electron-impact ionization of molecules. The purpose of this paper is to give an overview of that development and to provide a summary of the recent experimental and theoretical works examining low to intermediate energy electron-impact single ionization of molecules.

Electron-impact ionization of Xe24 +

Configuration-average distorted-wave calculations are carried out for the electron-impact single ionization of Xe24 +. Contributions are included from direct ionization of the 3s, 3p, 3d and 4s subshells and from indirect ionization via 3s → nl, 3p → nl and 3d → nl excitations followed by autoionization. Branching ratios are found for single versus double ionization of the 3s and 3p subshells and for autoionization versus radiative decay of all 3l → nl excitations. Additional distorted-wave and R-matrix calculations find resonant-capture double-autoionization contributions to be quite small. The total ionization cross section for Xe24 + is found to be dominated by indirect excitation–autoionization contributions, especially near the single-ionization threshold. An approximate 15% reduction in the total ionization cross section is found due to the radiative decays included in the branching ratios.

Electron impact ionization of xenon ions

We report on a comprehensive study of electron-impact single and multiple ionization of Xeq+ ions (q = 1, 2, ...,25) at energies from the observed thresholds up to 1000 eV. Special emphasis is on indirect processes and their contributions to the net ionization of the parent ion. Although this work is still in progress, it is clear that indirect processes dominate the cross sections. This is demonstrated for the example of single ionization of Xe22+ ions.

Exploring differences of atomic and molecular ionization by electron impact

Electron impact single ionization of helium and the hydrogen molecule is studied at equivalent electron kinematics to explore the differences of atomic and molecular ionization. Fully differential cross sections (FDCS) were obtained covering the whole solid angle in order to critically test theoretical calculations.

Near-threshold electron impact ionization of Ne and Xe

Doubly differential cross-sections for the single electron impact ionization of Ne and Xe have been measured at several energies below the second ionization energy. The results indicate that the ionization of Ne is strongly influenced by the polarization of the ionized 22P3/2,1/2 core, where as this influence is significantly reduced for Xe. Single differential cross-sections are derived from the doubly differential cross-sections and for Xenon these show profiles similar to Helium ("smile"), whereas for Neon they show a dissimilar profile ("frown").

The 5p6 autoionization cross section of cesium atoms: contribution to single ionization by electron impact

The autoionization cross section of cesium atoms in the incident electron energy range from the 5p6 lowest autoionization threshold at 12.3 eV up to 16.6 eV was investigated for the first time. The data were obtained by summarizing the normalized intensities of lines in ejected-electron spectra observed at the ‘magic’ angle of 54.7°. The autoionization cross section possesses a clear resonance character with a maximum value of 4.8 × 10−16 cm2 at 14.7 eV. By analysing the calculated multiconfiguration Dirac–Fock energies, decay channels and excitation cross sections for the lowest autoionizing configurations it was found that the quartet levels 5p55d6s give the bulk of the autoionization cross section and, therefore, of the autoionization contribution to the single ionization cross section of cesium. Both resonance shape and ‘fine’ structure of the autoionization cross section are caused by the presence of strong negative-ion resonances in the excitation of the levels in configurations 5p56s2, 5p55d6s and 5p56s6p. A comparison of the data is made with available experimental and theoretical electron impact single ionization cross sections of cesium atoms.

The effect of dynamical screening on helium (e, 2e) fully differential cross-sections

This paper presents the fully differential cross sections (FDCS) for 102eV electron impact single ionization of helium for both the coplanar and perpendicular plane asymmetric geometries within the framework of dynamically screened three-Coulomb-wave theory. Comparisons are made with the experimental data and those of the three-Coulomb wave function model and second-order distorted-wave Born method. The angular distribution and relative heights of the present FDCS is found to be in very good agreement with the experimental data in the perpendicular plane geometry. It is shown that dynamical screening effects are significant in this geometry. Three-body coupling is expected to be weak in the coplanar geometry, although the precise absolute value of the cross section is still sensitive to the interaction details.

Importance of polarization effects in electron impact single ionization of argon atom

We report the results of our calculations of triple differential cross section (TDCS) for electron impact single ionization (i.e. (e, 2e) processes) from the 3s shell of argon using a modified distorted wave Born approximation formalism by including correlation–polarization potential, which accounts for both correlation and polarization effects. We observe that DWBA formalism including polarization potential is able to reproduce most of the trends of experimental data and hence provide a fututre direction for further investigation of ionization process from the 3s shell of argon. We also compare our results with the available theoretical and experimental results. The present calculations significantly improve the agreement with the experimental results but still there are certain discrepancies, which is a matter of further investigation.

Triple differential cross sections for the electron-impact ionization of helium at 102 eV incident energy

We examine the time-dependent close-coupling (TDCC) approach to electron-impact single ionization of helium and study the convergence properties of our method. As an example, we compare our calculations to recent measurements of the triple differential cross sections from He after 102 eV electron impact, made for asymmetric electron energies and a variety of electron geometries. We find that our calculations compare well to the measurements and to convergent close-coupling calculations.

Near-threshold electron impact doubly differential cross sections for the ionization of neon and xenon

Normalized doubly differential cross sections (DDCSs) for the electron impact single ionization of Ne and Xe are presented. The Ne measurements are taken at incident energies of 23.5 eV, 25 eV, 30 eV and 40 eV while the Xe measurements are taken at 14 eV, 15 eV and 20 eV. Scattering angles in the experiment range from 15° to 120°. The measurements use a moveable target method for an accurate determination of the experimental background. Normalization of the differential data is initially made to available experimental cross sections for excitation of the ground np6 to the np5 (n + 1)s excited states of the noble gas and then, if necessary after integration, to available experimental total ionization cross sections. We show that our single differential cross sections, derived from integrating the DDCSs, show a convex profile (frown) for Ne whereas they are concave (smile) for Xe similar to what is observed for He and we suggest a tentative mechanism for this effect.

Electron-impact single ionization of Mg andAl+

Theory and experiment are compared for the electron-impact single ionization of Mg and ${\text{Al}}^{+}$. Nonperturbative $R$ matrix with pseudostates (RMPS) and time-dependent close-coupling (TDCC) calculations have been carried out that exhibit large reductions from perturbative distorted-wave results of 38% for Mg and 20% for ${\text{Al}}^{+}$. Experimental single-ionization data available for Mg and ${\text{Al}}^{+}$ are in reasonable accord with distorted-wave data and lie substantially above the new theoretical results. Rate coefficients, necessary for the collisional-radiative modeling of Mg and Al plasmas were generated from the RMPS ionization cross sections. In the collisional-ionization region near the ionization threshold, the resulting rates were found to be up to two times lower for Mg and three times lower for ${\text{Al}}^{+}$ than the rates generated from experimental data.

Calculated cross sections for the single ionization of atoms (N, Cu, As, Se, Sn, Sb, Te, I, Pb) by electron impact

By correcting some primary parameters in the semi-classical Deutsch–Märk (DM) formula, this paper calculates the absolute single electron-impact ionization cross sections of atoms N, Cu, As, Se, Sn, Sb, Te, I and Pb from threshold to 10000 eV. The calculated cross sections are compared with available experimental data and other theoretical results. An excellent agreement was achieved between the calculated and measured cross sections for these atoms over a wide range of impact energies.

Electron impact ionization of helium isoelectronic systems

The electron impact single ionization cross sections, on the helium isoelectronic He, Li1+, B3+, C4+, N5+, O6+, Ne8+, Na9+, Ar+16, Fe24+, Mo41+, Ag45+, and U90+ targets, are calculated modifying the simplified Bell (SBELL) model [Eur. Phys. J. D 46, 281 (2008)]. The results of the present analysis are compared with the available experimental and theoretical data. The modified SBELL (MSBELL) model, incorporating the ionic correction factor in it, produces excellent agreement with the experimental data and theoretical calculations for all the two-electron systems, neutral or ions. This model may be a prudent choice in plasma modeling due to its simple inherent structure.

Electron-impact ionization of singly-charged neon ions

Absolute cross sections for electron-impact single and multiple ionization of Ne+ leading to the formation of Neq+ (q = 2–5) are reported. The animated crossed beams method is applied in the energy range from the respective thresholds up to 2.5 keV. The maximum cross sections are found to be (4.12 ± 0.21) × 10−17 cm2, (1.42 ± 0.07) × 1018 cm2, (2.57 ± 0.14) × 10−20 cm2, (8.78 ± 0.54) × 10−22 cm2 for the multiply-charged products Neq+ (q = 2–5), respectively. The corresponding threshold energies are measured to be (41.0 ± 0.5) eV, (105 ± 1) eV, (205 ± 5) eV and (345 ± 15) eV. Present data are compared with available experimental and theoretical results.

Electron‐Impact Ionization of Be‐like Ciii, Niv, and Ov

We present recent measurements of absolute electron-impact ionization cross sections for Be-like C III, N IV, and O V forming Li-like C IV, N V, and O VI. The measurements were taken using the crossed-beams apparatus at Oak Ridge National Laboratory. A gas cell beam attenuation method was used to independently measure the metastable fractions present in the ion beams. The measured ionization cross sections were compared with calculations using the R-matrix with pseudostates and distorted-wave theoretical methods. Best agreement is found with the R-matrix with pseudostates cross sections results that account for the metastable fractions inferred from the gas attenuation measurements. We present a set of recommended rate coefficients for electron-impact single ionization from the ground state and metastable term of each ion.