Electron Impact Ionization Of Atoms Research Articles

Generalized binary-encounter-Bethe model for electron impact ionization of atoms

A generalized binary-encounter-Bethe (GBEB) model is proposed to calculate the partial ionization cross sections of all shells. The present model improves the original version of Kim et al (2000 Phys. Rev.A 62 052710) by incorporating a physically constructed effective charge felt by the ejected electron in the empirical factor, which prevents the selection of specific factors for different shells. A generalized relativistic binary-encounter-Bethe (BEB) formula is also proposed and applied to different inner shells of C, Al, Fe, Ar, Ag, Xe, Sn, Pb, and Bi atoms for impact energies from the thresholds up to 106 keV. The present model improves the partial ionization cross sections in the low-energy region compared to other relativistic BEB models. The GBEB partial and total ionization cross sections of the Xe atom are compared with the original BEB results. The present calculations, combined with the contribution from the direct multiple ionization, show good agreement with the experimental measurements in the intermediate- and high-energy ranges. We conclude that the present GBEB model, without any fitting parameters and ad hoc corrections, improves the BEB prediction of partial and total ionization cross sections for a good variety of atomic targets.

Electron impact ionization of atoms and molecules: an improved BBK model

We present a new model to study the dynamics of electron impact ionization of atoms and molecules. In this model, the two outgoing electrons are both described by a Coulomb wave with variable charges Z(r) while the incident electron is described by a plane wave. The fully differential cross-section is then calculated in (e,2e) reactions for several atomic and molecular targets taking into account the post-collision interaction. Calculations are carried out for the coplanar asymmetric kinematics, and the results are compared with those from available experiments.

Analytical Cross Section Approximation for Electron Impact Ionization of Alkali and Other Metals, Inert Gases and Hydrogen Atoms

The paper presents an analysis of data on the cross sections of electron impact ionization of atoms of alkali metals, hydrogen, noble gases, some transition metals and Al, Fe, Ni, W, Au, Hg, U. For the selected sets of experimental and theoretical data, an optimal analytical formula is found and approximation coefficients are calculated. The obtained semi-empirical formula reproduces the values of the ionization cross sections in a wide range of energies with an accuracy of the order of error of the available theoretical and experimental data.

Electron-ion momentum vector coincidences in electron collisions with atoms and molecules

We review experimental studies of electron impact ionization of atoms and molecules which became feasible by applying the electron-ion momentum vector coincidence technique. Besides briefly discussing excitation of metastable helium we present studies of single ionization of helium and argon and also mention a pilot experiment for positron impact ionization. The main strength of the method is revealed if three or more particles are to be detected and if the cross section of the reaction is small. Exemplarily helium double ionization close to threshold is shown. Studies of molecular ionization reveal a molecular alignment dependent collision dynamics for the molecular hydrogen target. For larger molecules ionized electronic orbital and ionic fragment can be correlated.

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 Schrodinger 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 Schrodinger 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.

Double Differential Cross-Sections for Electron Impact Ionization of Atoms and Molecules

The single ionizing collision between an incident electron and an atom/molecule ends up two kinds of outgoing electrons called scattered and ejected electrons. As features of electron impact ionization, these two types of electrons are indistinguishable. Double differential cross-sections (DDCS) can be obtained by measuring the energy and angular distributions of one of the two outgoing electrons with an electron analyzer. We used He, Ar, H2, and CH4targets in order to understand the ionization mechanisms of atomic and molecular systems. We measured differential cross-sections (DCS) and double differential cross-sections at 250 eV electron impact energy. The elastic DCSs were measured for He, Ar, H2, and CH4, whereas the inelastic DCSs of He were obtained for 21P excitation level for 200 eV impact electron energy.

Three-dimensional cross sections for electron impact ionization of atoms and molecules

Using a multi-particle momentum spectrometer (reaction microscope), three-dimensional and fully differential cross sections (FDCS) for electron impact ionization are obtained, providing benchmark data for comprehensive tests of theoretical calculations. Since all final-state particles, including the scattered projectile were detected, a good momentum transfer resolution was obtained also for heavy targets like Ar. Results for ionization of the Ar 3p- and He 1s-orbitals by 200 eV electron impact are presented. The cross section patterns display rich structure, which is partially reproduced by theory, although differences persist out-of-the scattering plane. Kinematically complete experiments for electron impact ionization of simple diatomic molecules have attracted increased attention concerning molecular structure effects. (e, 2e) on He and H2 is studied at equivalent collision kinematics to explore the differences of atomic and molecular ionization. Here FDCS were obtained covering the whole solid angle.

Rule of electron-impact ionization for the damage of C, N and O clusters irradiated by XFEL

Single differential and integral cross sections in electron-impact ionization of atoms and ions of C, N and O are calculated with the binary encounter dipole model (Kim Y-K. and Rudd M. E., 1994 Phys. Rev. A, 50 3954) to study electron-energy distribution in the C, N and O clusters irradiated by an X-ray free electron laser (XFEL). The damages to the C, N and O clusters irradiated by XFELs are also investigated with time-dependent rate equations, considering photo-ionization, Compton scattering, Auger decay and electron-impact ionization of the atoms and ions of C, N and O. We found that the electron-impact ionization is important process to estimate the damage of carbon clusters irradiated by XFEL.

Electron-impact ionisation of atoms and ions

We report on the recent applications of the convergent close-coupling method to electron impact ionisation of atoms and ions. Total and fully differential cross sections are considered. Some excellent agreement with experiment is identified, but some long-standing discrepancies still remain.

Constant of electron impact ionization in a weakly ionized argon plasma in crossed E×B fields

A theoretical model is proposed which allows one to calculate the electron energy distribution function, the average electron energy, and the constant of electron impact ionization of atoms for an argon plasma. The model considers both elastic and inelastic collisions that determine the electron energy distribution in crossed E×B fields. It is shown that the electron energy distribution is strongly different from the Maxwellian one in the case of no magnetic field, whereas in the cases of a strong magnetic field it asymptotically approximates the Maxwellian distribution.

Hartree–Fock results for electron impact ionization of H atoms

The multiconfiguration Hartree–Fock method for bound and continuum wavefunctions has been extended to investigate electron impact ionization of atoms. As a test case, the method is applied to calculate triple differential cross sections for electron impact ionization of H atom at an excess energy of 4 eV for the coplanar θ12 = π geometry. The results are obtained for both equal and unequal energies of the two outgoing electrons in the Hartree–Fock (HF) and screening potential (SP) approximations in order to determine the importance of electron correlation between the two outgoing electrons in the present configuration. At equal energies, both the HF and SP results are found to be very close to each other and are in very good agreement with the relative experimental measurements and other accurate theoretical calculations. At unequal energies, both calculated results also compare well with the relative experimental data over all ranges of θ1 considered by experiment.

Electron impact ionization of atoms at large momentum transfer: Renormalized plane wave first-order models

We consider electron impact single ionization of an atom near the Bethe ridge in the regime of large energy and momentum transfer. For evaluation of the ionization amplitude, a plane wave Born series is employed. A regularization procedure is utilized in circumventing typical divergences of the higher-order Born terms. The regularized Born series for the ionization amplitude is derived and, on this basis, renormalized analogues of the traditional plane wave Born and impulse approximations are developed. Numerical results using different approximations are presented and analyzed for the case of electron-hydrogen ionization.

Exchange distortion and postcollision interaction for intermediate-energy electron-impact ionization of argon

Measurements of fully differential cross sections for electron impact ionization of atoms have been performed for over 30 years. However, only within the last ten years has agreement between experiment and theory been achieved for ionization of hydrogen and helium. For the heavier inert gases, reasonably good agreement between experiment and theory has only been achieved for high incident energies while serious discrepancies are common for intermediate and low incident energies. It is believed that a major source of the problem stems from an improper/inadequate treatment of exchange distortion (ED) and the effects of post-collision interactions (PCIs). In this paper, two different methods for including ED are examined---one based upon the $R$ matrix (close-coupling) approach and one originating from the single-configuration Hartree-Fock approach. In general, these two methods predict significant, but different, ED effects. The importance of PCI is studied by including the final-state Coulomb interaction directly in the final-state wave function. This procedure guarantees that PCI effects will be included to all orders of perturbation theory. For intermediate energies, PCI is an important effect and leads to an overall improvement in the agreement between experiment and theory.

Parallel data acquisition system for electron momentum spectrometer

A parallel data acquisition system has been developed for the study of electron impact ionization of atoms and molecules. The system has a large data storage capacity providing good experimental resolution and system flexibility. The system is used to collect and analyze data from electron momentum spectroscopy experiment. Results from electron momentum spectroscopy experiments on C 4H 10 molecules, at an incident energy of 1200 eV, are presented to demonstrate the performance of the system.

Electron-impact ionization of atoms and ions from hydrogenic nl states

The dipole Bethe logarithmic constants for high-energy ionization cross sections of atoms and ions by electron impact have been calculated using the relation between and the bound-free Gaunt factor . By this method the values of are expanded to n = 50 and the constants calculated previously for the states 1s have been improved.

Data acquisition system for (e,2e) coincidence experiments

A data acquisition and display system has been developed for the study of electron impact ionization of atoms. Parallel data recording permits simultaneous but independent acquisition, analysis, and display of electron energy-loss spectra from two 180° electron energy spectrometers at single-detector count rates up to 100 kHz as well as their time coincidence spectra for coincidence rates of up to 1 kHz. The system has sufficient energy resolution to clearly observe the post-collision interaction energy shift of argon L23M23M23 Auger transition lines for low excess energy. It can acquire data at excess energies simultaneously from threshold to 30 eV for more than one transition, representing an improvement in experimental efficiency of at least an order of magnitude. Real-time data processing allows for faster experimental optimization and better statistical accuracy of accumulated data. Object-oriented programming in Windows with an IBM 486 computer was used.

Electron-impact ionization of atoms, molecules, ions and transient species

This topical review examines the current status of the electron impact ionization of atoms, molecules, ions and transient species such as free radicals and atoms/molecules in metastable states from the point of a view of the relevance of these processes for the physics and chemistry of low-temperature and moderate-temperature plasmas. The primary emphasis of this review is on recent developments in the measurement (and to a lesser extent also in the calculation) of absolute total and partial angle-integrated electron impact ionization cross sections. An attempt is also made to identify the most promising directions for future research in this field.

Ionization by electron impact

Electron impact ionization is-besides photon and ion impact ionization-the major ionization mechanism in natural and laboratory plasmas. Quantitative information on the kinetics of electron impact ionization of atoms, molecules and clusters (i.e. ionization cross sections) is therefore of the utmost importance for the understanding of this state of matter. Knowledge concerning experimental and theoretical determination of ionization cross functions is summarized.

Ionization by electron impact

Electron impact ionization is-besides photon and ion impact ionization-the major ionization mechanism in natural and laboratory plasmas. Quantitative information on the kinetics of electron impact ionization of atoms, molecules and clusters (i.e. ionization cross sections) is therefore of the utmost importance for the understanding of this state of matter. Knowledge concerning experimental and theoretical determination of ionization cross functions is summarized.

The effects of optical pumping on laser enhanced electron impact ionization of atoms

Excitation of an atom by optical radiation reduces the atomic ionization energy thereby increasing its electron impact ionization cross section. Since the enhancement in the ionization rate depends on the population of atoms that can be maintained in the excited state, optical pumping of the ground state hyperfine levels can have pronounced effect on the magnitude of the observed signal. Here, experiments with Na atoms pumped by a narrowband dye laser are reported. Optical pumping is shown to have a significant effect on the signal-to-noise ratio in transitions that do not constitute a two level system.