Direct Ionization Cross Sections Research Articles

Electron-impact ionization cross sections and rates for ions of argon

A distorted-wave Born exchange (DWBE) approximation including relativistic correction is used to calculate the electron-impact ionization cross sections and rate coefficients for the highly charged ions Ar 7+,…,Ar 17+. The comparison of the calculated results with the experimental data and other theoretical calculations shows that the DWBE method is valid for these ions of argon. The calculated results for direct ionization cross sections and excitation autoionization were fitted by empirical formulas to meet the requirements of applications. A set of improved empirical formulas are used for the fast and accurate calculations of rate coefficients from the fit parameters of cross sections.

Ionization of carbon, nitrogen, and oxygen by electron impact

Total ionization cross sections of neutral carbon, nitrogen, and oxygen atoms by electron impact are presented. In our theory we have included the possibilities that (a) some target atoms used in an experiment were in metastable states close to the ground state, (b) excitation-autoionization of ${2s2p}^{m}$ exited states may be substantial, and (c) ions produced in experiments may be in excited, low-lying metastable states. The binary-encounter Bethe (BEB) model of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)] is used to calculate the cross sections for direct ionization. Plane-wave Born cross sections scaled by the method developed by Kim [Phys. Rev. 64, 032713 (2001)] are used to determine the contributions from excitation-autoionization. A sum of the BEB cross sections for direct ionization weighted according to the statistical weights of the final ion states is used to modify the direct ionization cross sections. The combination of the BEB model and the scaled Born cross sections is in excellent agreement with available experimental data. The present method can easily be extended to heavier open-shell atoms.

On the normalization of the positron-impact direct ionization cross-section in the noble gases

Difficulties in the normalization of the positron-impact direct ionization cross-sections due to differences among the various data for electron impact ionization are discussed. Consequently, the cross-sections of Kara et al. (J. Phys. B 30 (1997) 3933) and of Moxom et al. (Can. J. Phys. 74 (1996) 367) have been renormalized using more recent and accurate data for electron impact ionization from Sorokin et al. (Phys. Rev. A. 58 (1998) 2900; 61 (2000) 022723). The new normalization for Ne, Kr and Xe lowers the cross-sections by 19%, 8% and 14% respectively, while that for Ar increases the cross-section by 2%.

Ionization of boron, aluminum, gallium, and indium by electron impact

Measurements of electron impact ionization of neutral Al, Ga, and In show large cross sections compared to other elements in the same rows of the periodic table. Semiempirical and classical calculations of direct ionization cross sections are all substantially smaller. Calculations by McGuire [Phys. Rev. A 26, 125 (1982)] for aluminum that include excitations to autoionizing ${3s3p}^{2}$ doublet levels are 2.5 times higher than experiment at the peak. We report the direct ionization cross sections based on the binary-encounter-Bethe model of Kim and Rudd [Phys. Rev. A 50, 3954 (1994)], which is an ab initio theory. We add the autoionization contribution using scaled plane-wave Born cross sections as recently developed by Kim [Phys. Rev. A 64, 032713 (2001)] for excitations to the first set of autoionizing levels. Dirac-Fock wave functions are used for the atomic structure. Our results are in excellent agreement with experimental values and support substantial contributions from excitation-autoionization to the total ionization cross sections for these elements. We also compare the total ionization cross section of boron to available theories, though no experimental data are available.

ELECTRON IMPACT IONIZATION CROSS SECTIONS FOR THE Na-LIKE IONS

By using a distorted-wave Born exchange ( DWBE ) approximation method including the relativistic correction, we calculated the direct ionization and excitation autoionization cross sections of eight Na-like ions. Based on the calculated results, we present two empirical formulas with high accuracy for calculating direct ionization and excitation autoionization cross sections. Our results are in good agreement with the experiment data and other theoretical values.

Empirical formula for cross section of direct electron-impactionization ofions

Plasma ionization composition and level populationcalculations require, inparticular, the cross sections of direct ionization from each quantumstateinto each state which may be generated by means of removal of anyelectron.We analysed published data and propose here an empirical formula forcross sections of direct electron-impact ionization of positive atomicions.The cross sections given by this formula are in satisfactory agreementwiththose calculated in the distorted-wave (DW) approximation;therefore, we believe that for any direct state-to-stateionization channel this formula provides a reasonably accurateprediction of the DW result. Comparisons with published data and withthe Lotz formula are reported as well.

Determination of partial single differential electron ionization cross sections of H2

The partial single differential cross sections (PSDCS) for the ions H2+ and H+ resulting from the direct and dissociative ionization of H2 by electron impact have been calculated employing a semi-empirical approach. The calculations are made as a function of the secondary/ejected electron energy at 500 and 1000 eV incident electron energies. The partial ionization cross sections (PICS) for direct and dissociative ionization have also been calculated with incident electron energies varying from the ionization threshold to 10 keV. The results are compared with the available experimental data.

Electron-impact single ionization of Kr10+ and Kr11+ ions

Cross sections for electron-impact single ionization of ground state Kr10+ and Kr11+ ions have been calculated in the distorted-wave approximation including both direct ionization and excitation-autoionization processes. The direct ionization cross sections are calculated for the ejection of 3s, 3p and 3d subshell electrons and the excitation-autoionization cross sections are calculated for the inner-shell excitations of the type 3p nl (n = 4,5,6;l = 0,1,2,3) and 3s n´l´ (n´ = 4,5;l´ = 0,1,2,3). In addition, the inner-shell excitation of the type 2p3d, 4l (l = 0,1,2,3) is included for Kr11+. The radiative decay effect has been found to be negligible by carrying out explicit calculations of the radiative rates and autoionizing rates. The present results show that the direct process dominates the total ionization cross section for the two ions and that the contributions of excitation-autoionization are about 15% and 13% of the total cross sections for Kr10+ and Kr11+ ions, respectively. The present results are in good agreement with the experimental measurement of Oualim et al.

Total, direct and dissociative electron impact ionization cross sections of the acetylene molecule

The total electron impact ionization cross sections of the acetylene molecule have been measured in the incident electron energy range from threshold to 1000 eV. These results are compared with other existing data, obtained by direct measurements of this molecular property or by indirect ones, via the partial ionization cross sections for the formation of different ions. Using three semiempirical equations, the total ionization cross sections were calculated and compared to available data, too. Direct and dissociative ionization cross sections were also calculated by a semiempirical equation and compared to existing data.

Non-dissociative and dissociative ionization of CO, CO2 and CH4 by positron impact

Cross sections for direct and total non-dissociative ionization of CO, CO2 and CH4 by positron impact have been measured for projectile energies ranging from threshold to 2000 eV. For CO and CO2 direct and total dissociative cross sections for several different fragments have been measured. The non-dissociative cross sections are observed to behave similarly to the single ionization cross sections of the light noble gases. The dissociative cross sections resemble that seen recently for N2, and can be related to models for double ionization of the light noble gases.

Electronic versus vibrational excitation in He q+ collisions with fullerenes

Collisions of He q+ ions with neutral fullerenes have been studied as a function of projectile velocity ( v ≈ 0.1–1.0 a.u.) and charge state ( q = 1, 2). With increasing velocity, two trends are observed for both charge states: The yield of C 60−2 m r+ clusters decreases with 1/ v, as expected for quantities related to direct vibrational excitation. The relative cross section for multifragmentation increases linearly with v and can be associated with electronic excitations. The additional potential energy of He 2+ with respect to He + manifests in increased direct ionization cross sections (high v) and multifragmentation cross sections (low v), respectively, revealing information about coupling times between electronic and vibrational excitation.

RATE COEFFICIENTS OF ELECTRON IMPACT IONIZATION FOR HIGHLY IONIZED IONS

In this work, some fitting formulas for the accurate and fast calculation of rate coefficient are given. Some fitting parameters of direct ionization cross sections and excitation autoionization cross section are also provided. The rate coefficients of some H-like, He-like and Li-like ions are calculated.

ENERGY-LOSS EFFECT IN K-SHELL IONIZATION

The energy-loss effect of the projectile for direct inner-shell ionization cross sections by charged-particle impact has been examined. The relativistic and nonrelativistic calculations for K-shell ionization with and without the energy-loss effect are made in the plane-wave Born approximation and compared with the Brandt-Lapicki theory for the corrections of the relativistic and energy-loss effect. It is demonstrated that the Brandt-Lapicki method gives a good approximation to both relativistic and nonrelativistic cross sections, which implicitly take into account the energy-loss effect. However, the use of the Brandt-Lapicki relativistic correction method in the nonrelativistic theory with the exact integration limits for energy and momentum transfer overestimates the relativistic calculations for low-energy projectiles. This indicates that the Brandt-Lapicki method for correction of the electronic relativistic effect should be used only with their energy-loss correction method.

Excitation and ionization of neutral Cr and Mo, and the application to impurity influx

The atomic physics required to study impurity influx from a localized surface is examined and then applied to chromium and molybdenum. To this end, we have calculated term energies and oscillator strengths for the and transitions in neutral Cr and Mo in a multiconfiguration Hartree - Fock approximation. We have calculated excitation cross sections for these dipole transitions and for the transitions, which couple the ground to the metastable system, in a close-coupling approximation using the non-relativistic R-matrix method. Pseudo-resonances were removed from the latter transitions using a novel transformation and reduction method which eliminates those linear combinations of (N + 1)-electron bound states that are not required by orthogonality. We have calculated the direct ionization cross sections from the ground and metastable terms in a distorted-wave approximation, and find significant near-threshold sensitivity to the effective potentials used. We have incorporated this primary atomic data into the Atomic Data and Analysis Structure (ADAS) and solved the quasi steady-state generalized collisional - radiative population rate equations. We present results for the number of ionization events per observed photon, the ratio, over a wide range of electron temperatures and densities. The ratio can be used to directly relate spectroscopic emissivity measurements to impurity flux from a localized surface such as the target plates in a magnetic fusion divertor.

Single ionization by positron impact

The single direct ionization cross sections for positron scattering [Formula: see text] on He, Ar, Kr, and H2 were measured and compared with existing data and the corresponding cross sections for electron impact [Formula: see text]. At most impact energies the present data for He and H2 are in reasonable accord with other measurements and, in the case of He, with some of the available calculations. At low energies, [Formula: see text] is found to increase more slowly than [Formula: see text], probably due to the importance of Ps formation in this energy range.

The configuration-interaction effects on the excitation - autoionization cross sections of the Na-like Cu ion

The configuration-interaction effects on the excitation - autoionization cross sections for the Na-like Cu ion from (l = 0,1,2) transitions were investigated in some detail. The excitation cross sections were calculated using the distorted-wave Born exchange approximation (DWBE) and configuration-interaction, bound-state wavefunctions. The autoionizing branching ratios were calculated without and with configuration interaction. The calculated results indicated that, the configuration interaction can increase the excitation - autoionization cross sections for the transition by about a factor of two, and reduce the autoionization cross sections for by about 35% but only has a slight effect on the autoionization cross sections for the transition. In addition, the autoionization cross sections make a substantial contribution to the total electron-impact ionization cross sections, it is about a factor of four of the direct ionization cross sections.

Direct ionization in the quasimolecule H-He.

Doubly differential cross sections for electron ejection in H-He collisions have been measured in the incident atom energy range 2.3--10 keV. Analysis of the experimental data on the basis of the adiabatic theory of direct ionization has been made and parameters of the quasimolecule, namely, the energy of the initial 2p\ensuremath{\sigma} state coupled with the continuum, the effective charge, and the real and imaginary parts of the internuclear distance at which the ``superpromoted'' diabatic term crosses boundary of continuum, have been determined. The values of the initial-state energy and the effective charge agree well with the corresponding parameters of the Li united atom, whereas the dynamical characteristics ReR(E) and ImR(E) reveal considerable influence of electron correlations. The possibility for the development of an alternative method of quantitative spectroscopy of quasimolecules based on the analysis of experimental data on differential cross sections for direct ionization is discussed. \textcopyright{} 1996 The American Physical Society.

Absolute electron impact ionization cross sections for N2O and NO from threshold up to 1000 eV

Cross sections for the production of singly charged ions by electron impact on N2O and NO have been obtained by utilizing the relative flow technique in which the accurately known values of cross sections for the production of singly charged ions of rare gases have been used for normalization. By summing the cross sections for direct ionization and dissociative ionization, the total ionization cross sections have been obtained and compared with the previously available data. Energies for the appearance of various ions have also been measured and compared with previous determinations from photoelectron spectroscopy.

Absolute cross-section measurements for electron-impact single ionization of Se + and Te +

The crossed-beams technique has been used to measure absolute cross-sections for the single ionization of Se + and Te + at electron energies from threshold to 200 eV. The peak cross-sections obtained are 16.6 × 10 −17 cm 2 at about 53 eV for Se + and 24.3 × 10 −17 cm 2 at about 43 eV for Te +. The measured Se + cross-sections are bracketed from above by direct-ionization cross-sections from configuration-averaged distorted-wave calculations of Pindzola [unpublished work] and from below by results using the single parameter semi-empirical formula of Lotz [Z. Phys., 216 (1968) 241], with both calculations giving results in moderate agreement with the measurements. In contrast, comparisons of measured Te + cross-sections with similar calculations are generally poor. Ionization-rate coefficients for a Maxwellian electron-temperature distribution calculated from the experimental data are presented.

Single and doubleK-shell ionization and electron-transfer cross sections for Fe and Ni bombarded by S ions and Fe by Si ions at 1.25–4.70 MeV/amu

Single and double K-shell vacancy production and K-K electron-transfer cross sections have been measured in the limit of zero target thickness for Fe and Ni induced by 1.25--4.70 MeV/amu $^{28}\mathrm{Si}$ and $^{32}\mathrm{S}$ ions. The fluorescence yield ${\mathrm{\ensuremath{\omega}}}_{\mathit{k}}$ for Fe and Ni at x-ray emission was determined from the measured energy shifts of target K x rays, the intensity ratios of K\ensuremath{\beta} and K\ensuremath{\alpha} and using the statistical scaling procedure of Larkins [J. Phys. B 4, L29 (1971)]. The single-electron-transfer cross sections are compared with the atomic-orbital-expansion model (AO) of Fritsch and Lin [Phys. Rep. 202, 1 (1991)], and the semiempirical perturbed-stationary-state (PSS) approach of Lapicki and McDaniel [Phys. Rev. A 22, 1896 (1980)]. The AO calculations show excellent agreement with the single K-K transfer cross sections. This model also explains the double K-K transfer data quite well although it underestimates slightly the measured values. The direct K-shell ionization cross section data are compared with the ECPSSR (perturbed-stationary-state theory with energy-loss, Coulomb deflection, and relativistic corrections) calculations.