Distorted wave cross sections for electron-impact ionization of the lithium-like ions

Electron-impact ionization cross sections for lithium-like ions have been computed in three variants of the distorted wave approximation. It is found that the inclusion of the electron exchange in the standard form yields the best results, in very good agreement with the experiment and the elaborate calculations of Jakubowicz and Moores.     

Geometrical model approximation for ionization cross sections of atoms and ions in ion-atom collisions

The applicability of the geometrical model (GM) proposed in [14, 15] to calculate the ionization probability and cross section of atoms and ions during collisions with ions, including structural ions, was studied at high velocities of partners. The GM represents a version of the classical impulse approximation. It was shown that the GM in all cases under consideration allows estimation of the ionization probability and total corss section of an atom or an ion during collisions with ions with deviations of no more than 50% from the results of the classical Monte Carlo method. GM calculations are rather simple in computation respect and limit the ionization probability by unity at arbitrary impact parameters.     

Electron-impact ionization cross section calculations for lithium-like ions

The electron-impact ionization of lithium-like ions C³⁺,N⁴⁺,O⁵⁺,Ne⁷⁺,and Fe2³⁺is studied using a combination of two-potential distorted-wave and R-matrix methods with a relativistic correction.Total cross sections are computed for incident energies from 1 to 10 times of ionization energy and better agreements with the experimental results are obtained in comparison with the theoretical data available.It is found that the indirect ionization processes become significant for the incident energy larger than about four times of the ionization energy.Contributions from the exchange effects along the isoelectronic sequence are also discussed and found to be important.The present method can be used to obtain systematic ionization cross sections for highly charged ions across a wide incident energy range.     

电子与高电荷离子碰撞的细致能级电离截面的相对论计算

电子离子碰撞电离的细致能级截面在X射线激光等需要确定能级布居数的高技术研究中有非常重要的意义.本文给出了在全相对论框架下利用扭曲波波恩交换近似计算细致能级电离截面的方法.作为示例,计算了若干高电荷离子细致能级的电离截面,并分析了组态相互作用的影响.计算结果与其它理论值进行了比较,获得了较好的符合.     

Proton and α-particle impactM-shell ionization of atoms in binary encounter approximation

M-shell ionization cross sections for atoms due to the impact of proton and α-particles have been calculated in the binary encounter approximation. The effects of Coulomb deflection of the incident projectile and increase in binding of the target electron have been investigated. Roothan-Hartree-Fock velocity distribution for the target electrons has been used in the present work. The calculated cross-sections have been compared with experimental results and other theoretical calculations wherever available. The present calculations give a good account of experimental observations.     

Near-threshold electron-impact ionization of calcium atoms

The electron-impact ionization of calcium atoms is studied in the near-threshold energy range (from 6.11 to 16 eV). Experiments were performed by the method of intersecting electron and atomic beams with the recording of formed positive calcium ions. The electron beam (ΔE _1/2 = 0.15 eV) was formed using a hypocycloidal electron monochromator. An analysis of the specific features of ionization cross sections revealed a contribution from the excitation and decay of low-lying autoionization atomic states, which converge to the excitation thresholds of the 3d, 4p, and 5s ionic levels, and resonances (long-lived states of negative ions). The specific features of cross sections are identified using the experimental and theoretical data on photoionization (photoabsorption).     

Ratio of single K-shell ionization cross section to double K-shell ionization cross section in heavy-ion-atom collisions

The spectra of Ti Kα X-rays induced by He, C, N and O ions were measured with the use of a Bragg crystal spectrometer. The ratios of the integrated hypersatellite to diagram-plus-satellite X-ray yields were obtained. The previously obtained data on Cr, Fe and Ni Kα X rays induced by N ions were also analysed. It is found that the double K-shell vacancy production cross section is proportional to Z⁴₁.     

Classical binary-encounter collision theory for electron-impact ionization of ions

We calculated electron-ion impact ionization using classical binary-encounter collision theory. A differential cross section per unit energy and momentum transfer is derived for two-electron scattering. We obtain the total ionization cross section by integrating this differential cross section over the appropriate energy and momentum transfer and the bound electron-velocity distribution. To simplify the computation, we calculated the bound electron-velocity distribution in the Thomas-Fermi approximation. Comparison with other theoretical calculations shows that our results agree within 50% with the best quantum qpproximations. Our cross sections and rate coefficients, which are sufficiently accurate for many practical applications, are expressed in computationally simple form.     

Asymptotics of the cross section for the electron loss by atoms and ions in light media

The dependence of the cross section of electron loss on the nuclear charge Z of a bombarding ion and on the nuclear charge Z _t of a target atom for fast collisions is studied theoretically using the plane-wave Born approximation and the sum rule. The results of calculations show that the cross section of electron loss for fast collisions increases monotonically as Z and Z _t increase, which can be used to interpolate cross sections for the processes for which there are no experimental data.     

Relativistic calculations of electron-impact ionization for highly charged hydrogen-like ions

Electron-impact ionization cross-sections and rate coefficients of the 1s ground state for H-like C, O, Mg, Ar, Fe, Cu, As, Kr, Y, Mo ions with incident electron energies up to 15 times the ionization threshold energy have been systematically calculated by the relativistic distorted-wave Born exchange (DWBE) approximation. The comparison of the result with the experimental data, other theoretical calculations and recommended values shows the very good agreement. The influence from relativistic and the lowest order QED effect in the calculation is discussed. The calculated ionization cross-sections are fitted by empirical formulas. These fits can be readily integrated over a relativistic Maxwellian electron distribution function to obtain rate coefficient for plasma modeling.     

Modern methods for calculating photoionization and electron-impact ionization of two-electron atoms and molecules

A number of recently developed methods for calculating multifold differential cross sections for photoionization and electron impact ionization of atoms and molecules with two active electrons are reviewed. The methods are based on unique approaches to the calculation of three-body Coulomb wave functions. The exterior scaling method and the driven Schrödinger equation formalism are considered. The effectiveness of the time-dependent approaches to the scattering problem, such as the paraxial approximation and the time-dependent scaling, is demonstrated. A novel numerical method is formulated, which has been developed by the authors to solve the six-dimensional Schrödinger equation for an atom with two active electrons on the basis of the Chang-Fano transformation and the discrete variable representation. The threshold behavior manifested by the angular distributions of the two-electron photoionization of a negative hydrogen ion and a helium atom and the multifold differential cross sections for the electronimpact ionization of hydrogen and nitrogen molecules are analyzed on the basis of numerical simulations. The Wannier law for the angular distribution of double ionization is demonstrated to be incorrect even at very low energies.     

Multielectron ionization of atoms by fast ions: An approximation by normalized exponentials

Multielectron ionization of neutral atoms by fast positive ions is considered in terms of the independent particle model. A relatively simple technique for calculating the multielectron ionization probabilities and cross sections through the impact parameter is suggested in which one-electron ionization probabilities are represented as normalized exponentials p _nl(b) = p _nl(b) = p _nl(0)exp(?α_nl b), where b is the impact parameter and n and l are quantum numbers of the target atomic shell. Exponent α_nl is determined from the Born one-electron ionization cross section for target atoms, and preexponential p _nl(0) (the ionization probability at a zero impact parameter) is found from a geometrical model. This technique provides the normalization condition p _nl(b) ≤ 1 irrespective of the velocity and charge of striking ions and makes it possible to calculate the one-, two-, and three-electron ionization cross sections, which, when added up, make a major contribution to the total cross section, up to a factor of 2. The results of our computations are compared with experimental data and analytical results of other authors.     

Perturbative calculation of the cross section in double ionization by high-energy Compton scattering

In this paper we investigate double ionization in high-energy Compton scattering from the He-atom including both the shake-off mechanism and a perturbative correction to that mechanism. The correction is calculated in second-order perturbation theory and includes Coulomb electron-electron interaction in addition to the correlation in the ground state of the He-atom. Our calculations for the ratio of double to single cross section cover the range from 30 to 300 keV of impact photon energy and explain the slow convergence of the ratio towards the asymptotic value.