High-energy Ion-atom Collisions Research Articles

Continuum distorted-wave method in the two-electron charge exchange theory

Introduction. In the case of collisions with multi-charged ions of atoms, more complex than the hydrogen atom, other than single-electron charge exchange and ionization, other processes may occur. These are, first of all, the processes of two-electron capture, capture with simultaneous ionization or ion excitation and others. We shall stop on the problems of the theoretical description of the processes of two-electron capture in the region of intermediate and high collision velocities of ions with atoms. Purpose . Research the interactions of multi-charged ions with atoms in the region of intermediate and high collision velocities using theoretical methods. Methods. To describe the double charge exchange processes in high-energy ion-atom collisions a four-part formalism of continuum distorted-wave method (CDW-method), based on the integral Dodd-Graider's equations, were developed. Results. We obtained a matrix element that describes the two-step (Thomas) mechanism of electron capture through a continuous spectrum from a target atom into bound states relative to the fast -particle. For the reaction amplitude, in the approximation of the mechanism of simultaneous capture of two electrons, a general expression that takes into account Coulomb effects in the initial and final states is found. Conclusion. The application of the general theory is illustrated by the example of a two-electron charge exchange reaction in collisions of helium atoms with -particles. The results of calculations of the cross sections are in good agreement with the experimental and theoretical data. An attractive side of the proposed formalism is the consistent consideration of the asymptotics of wave functions in both channels of the reaction, which takes into account the long-range nature of the Coulomb interactions.

Four-body methods for high-energy ion-atom collisions

The progress in solving problems involving nonrelativistic fast ion (atom)-atom collisions with two actively participating electrons is reviewed. Such processes involve, e.g., (i) scattering between a bare nucleus (projectile) $P$ of charge ${Z}_{P}$ and a heliumlike atomic system consisting of two electrons ${e}_{1}$ and ${e}_{2}$ initially bound to the target nucleus $T$ of charge ${Z}_{T}$, i.e., the ${Z}_{P}\text{\ensuremath{-}}{({Z}_{T};{e}_{1},{e}_{2})}_{i}$ collisions; (ii) scattering between two hydrogenlike atoms ${({Z}_{P},{e}_{1})}_{{i}_{1}}$ and ${({Z}_{T},{e}_{2})}_{{i}_{2}}$, etc. A proper description of these collisional processes requires solutions of four-body problems with four active particles including two nuclei and two electrons. Among various one- as well as two-electron transitions which can occur in such collisions, special attention will be paid to double-electron capture, simultaneous transfer and ionization, simultaneous transfer and excitation, single-electron detachment and single-electron capture. Working within the four-body framework of scattering theory and imposing the proper Coulomb boundary conditions on the entrance and exit channels, the leading quantum-mechanical theories are analyzed. Both static and dynamic interelectron correlations are thoroughly examined. The correct links between scattering states and perturbation potentials are strongly emphasized. Selection of the present illustrations is dictated by the importance of interdisciplinary applications of energetic ion-atom collisions, ranging from thermonuclear fusion to medical accelerators for hadron radiotherapy.

Radiations in ion-atom collisions

After a short survey of the present state of the field of atomic and atomic collision physics the main types of radiation produced in fast ion-atom collisions and the associated instrumentation are described. Then some of the models for collision mechanism, which interpret the observed phenomena, are presented. Finally, examples are given of applications of the high-energy ion-atom collisions.

Alignment effects in fast ion-atom collisions

Angular distribution of Auger electrons and X-rays, and X-ray polarization in high energy ion-atom collisions, measured in the last few years, are reviewed. The excitation mechanisms of interest are the resonant and nonresonant transfer and excitation (RTE and NTE respectively), capture, ionization, excitation and transfer ionization. An alignment of the excited states formed through these excitation mechanisms is often observed and seems to be strongly dependent on the excitation mechanism.

Classical-trajectory Monte Carlo calculations for energetic electron-capture processes.

Electron-capture processes of a proton from a hydrogen atom are studied at a collision energy of 5 MeV by the classical-trajectory Monte Carlo method. Thomas double-scattering processes and knock-on captures, which are characteristic of high-energy ion-atom collisions, are detected in a two-dimensional treatment. Another type of capture that takes place through a momentum-matching mechanism is also investigated. Three-body effects modify these processes considerably in some cases from the idealized pictures given in a simplified treatment of successive two-body collisions published before [see, e.g., R. Shakeshaft and L. Spruch, Rev. Mod. Phys. 51, 369 (1979)]. Three-dimensional calculations designed for knock-on captures are also performed, and the cross section, 1.9\ifmmode\times\else\texttimes\fi{}${10}^{\mathrm{\ensuremath{-}}29}$ ${\mathrm{cm}}^{2}$, is in good agreement with quantum-mechanical calculations.

Auger emission and transfer ionisation in high energy Li3+ and He(H2) collisions - a QED approach

Ion induced Auger emission leading to transfer ionisation (ATI) in high energy ion-atom collisions is computed for Li3+ on H2 and He targets. QED computations for transfer ionisation (TI), charge transfer (CT) and double capture (DC) were recently reported in two papers (Bhattacharyya et al., 1988 [1], Bhattacharyya, 1989 [2]) subsequently referred to as paper I and II, respectively. Double capture of electrons leading to transfer ionisation ATI is here done in a similar technique. The cross section for ATI is found to lie between 0.1% to 0.2% of the sum of the cross sections for TI and ATI events.

Uncorrelated transfer excitation collisions at high energies.

A new mode of transfer excitation (TE) process in high-energy ion-atom collisions is analyzed in which two or more electrons in the target system are involved incoherently in the excitation of the projectile ion and electron transfer. This uncorrelated TE mode (UTE) is shown to resolve the long-standing discrepancy in the high-energy tail of the resonant TE (RTE) cross section in the ${\mathrm{Nb}}^{31+}$+${\mathrm{H}}_{2}$ collisions, and explains satisfactorily the recent experimental studies of the system ${\mathrm{F}}^{6+}$+${\mathrm{H}}_{2}$ by Schulz et al. [Phys. Rev. Lett. 62, 1738 (1989)] for the uncorrelated transfer excitation followed by x-ray decay plus RTEX, involving radiative decay to a singly excited state, and of the low-lying states of ${\mathrm{F}}^{6+}$ with He by Zouros et al. [Phys. Rev. Lett. 62, 2261 (1989)] for the UTE involving Auger decay.

Multiple-electron processes in fast ion-atom collisions

Research in atomic physics at the Lawrence Berkeley Laboratory SuperHILAC and Bevalac accelerators on multiple-electron processes in fast ion-atom collisions is described. Experiments have studied various aspects of the charge-transfer, ionization, and excitation processes. Examples of processes in which electron correlation plays a role are resonant transfer and excitation and Auger-electron emission. Processes in which electron behavior can generally be described as uncorrelated include ionization and charge transfer in high-energy ion-atom collisions. A variety of experiments and results for energies from 1 to 420 MeV u are presented.

The reduction of the systematic error caused by the nuclear background in high resolution electron spectroscopy of high-energy ion-atom collisions

The presence of background radiation emitted by the products of nuclear reactions is a source of systematic error in high resolution electron spectroscopy of high-energy ion-atom collisions. A detailed discussion of the role of the different background sources contributing to this systematic error is given. A method for decreasing this systematic error and its effect to the decomposition of electron spectra is presented.

The postcollision interaction observed at high-energy ion-atom collisions

The shape of the Ne KL 2L 3( 1D 2) Auger line has been investigated in the 5.5 MeV/u N 2+ — and Ne 3+ -Ne collisions. An asymmetric line shape has been found showing a tail on the high-energy side of the line, while the shape of the line due to the 1s2s2p( 4P)-1s 2( 1S) transition from the Li-like metastable initial state measured in the 5.5 MeV/u Ar 16+ -Ne collision was symmetrical. The line shape calculated according to a postcollision effect due to the slowly receding ejected electrons shows a very good agreement with the measured line shapes.

MULTIPLE CAPTURE AND IONISATION IN HIGH ENERGY ION-ATOM COLLISIONS

Multiple electron processes occuring in high energy ion-atom collisions are investigated through Independent Electron Models (IEM), some limits of which are indicated for double capture processes. It is shown that an IEM permits to exhibit non perturbative behaviours of the ionisation process. It appears to be a powerful tool to predict multiply charged recoil ion production and recoil energy distribution when the impact velocity of the primary beam is high enough.

Radiative processes in high-energy ion-atom collisions

Instantaneous photon emission in ion-atom collisions is considered from the standpoint of theoretical consistency. The processes discussed include electron bremsstrahlung, radiative electron capture, and atomic bremsstrahlung. Particularly detailed discussion is made on radiative electron capture. Formulas based on nonrelativistic quantum mechanics and on semiclassical theory are found to conflict with each other. This inconsistency is removed by taking the gauge-invariance requirement into consideration on the matrix elements of the interaction of the particle system with the radiation field. Finally, we present an approximation method that satisfies the gauge-invariance requirement. In addition, we show the results of calculations for radiative electron capture by Xe 54+ and by U 92+ from Be as examples and compare them with experiments as well as calculations based on existing theories.

Charge transfer and excitation in high-energy ion-atom collisions

Coincidence measurements of charge transfer and simultaneous projectile electron excitation provide insight into correlated two-electron processes in energetic ion-atom collisions. Projectile excitation and electron capture can occur simultaneously in a collision of a highly charged ion with a target atom; this process is called resonant transfer and excitation (RTE). The intermediate excited state which is thus formed can subsequently decay by photon emission or by Auger-electron emission. Results are shown for RTE in both the K-shell of Ca ions and the L-shell of Nb ions, for simultaneous projectile electron loss and excitation, and for the effect of RTE on electron capture.

Ionization and charge transfer in high-energy ion-atom collisions

Electron capture and loss by fast highly charged ions in a gas target, and ionization of the target by passage of the fast projectile beam, are fundamental processes in atomic physics. These processes, along with excitation, can be experimentally studied separately (“singles”) or together (“coincidence”). This paper is a review of recent results on singles measurements for electron capture and loss and for target ionization, for velocities which are generally high relative to the active electron, including recent ionization measurements for a nearly relativistic projectile.

Differential electron-capture cross-sections in high energy ion-atom collisions : comparison of experiment and theory for the Thomas peak

The recent experimental observation of the Thomas peak on the differential cross-section for high energy electron capture is confronted with the theoretical prediction. Very good agreement is obtained for all scattering angles with the CDW approximation including the internuclear interaction Bon accord a tous les angles de diffusion pour les collisions H + -He et H + -H 2 , en tenant compte de l'interaction entre noyaux a l'approximation CDW

Computation of total cross-sections for electron capture in high energy ion-atom collisions

Computation of total cross-sections for electron capture in high energy ion-atom collisions

Book review

Book review

Computation of total cross-sections for electron capture in high energy ion-atom collisions

Computation of total cross-sections for electron capture in high energy ion-atom collisions

Measurement of Charge Exchange Losses in the VICKSI Cyclotron

High-energy (E=5 to 20 MeV/amu) ion-atom collisions have been studied using the internal ion beam of the VICKSI separated-sector heavy-ion cyclotron. Various combinations of accelerated ions and target gases have been investigated using the beam attenuation method to infer charge-exchange cross sections as a function of ion energy and atomic target. The analysis indicates σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Δq</sub> ≈σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">q</sub> ß <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-1</sup> (where ß=v/c) for Z <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ion</sub> =6, 20 and 40 and E <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ion</sub> =0.5 to 20 MeV/amu in N <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> , Ar and Kr gas.

Distorted-wave Born approximation for inelastic collisions: Application to electron capture by positrons from hydrogen atoms

We have re-examined the distorted-wave Born approximation for inelastic collisions. We find that the distortion in the relative motion of the collision partners cannot be neglected even for high-energy ion-atom collisions. Furthermore, if the distortion in the relative motion is not treated exactly, post-prior discrepancies will occur. We have applied the distorted-wave Born approximation, with distortion included through first order, to ground-state-to-ground-state electron capture by positrons from hydrogen atoms. The results are presented in this paper. We have also examined the nonrelativistic asymptotic behavior of the cross section for electron capture from hydrogen by positrons incident with a speed $v\ensuremath{\sim}\ensuremath{\infty}$. We find that, for $\frac{{e}^{2}}{\ensuremath{\hbar}v}\ensuremath{\ll}1$, capture occurs primarily to states of positronium that have an odd orbital-angular-momentum quantum number. It follows that the cusp signifying charge transfer to the continuum will, for positron impact, be symmetric when $\frac{{e}^{2}}{\ensuremath{\hbar}v}\ensuremath{\ll}1$.