State-selective Electron Capture Research Articles

Cross sections for electron capture and excitation in proton collisions with the metastable Be(2s2p3P) atom

The two-center atomic orbital close-coupling method is employed to study electron capture and excitation processes in proton collisions with the metastable Be(1s2s2p P) atom. The interaction of the active electron with the Be ion core is represented by a model potential reproducing accurately (to within 3.5%) the energies of triplet excited states Be(1s2snl L) (at least up to n = 7). The excitation and state-selective electron capture cross sections up to the n = 5 shells of both centers are calculated in the energy range 1–200 keV/u using an expansion basis involving all the states with n ≤ 7 on H and all triplet states with n ≤ 7 on Be, augmented with a number of pseudostates. In the energy region below ~10 keV/u, the cross sections of both processes exhibit oscillatory structures, resulting from the multistate coupling accounted for in the dynamical model. It has been found that the magnitude of excitation cross sections above ~10 keV/u is dominated by the multipole interactions, while below this energy the excitation process proceeds through intermediate electron capture states.

State-selective electron capture in collisions of C6+, N7+ with Hydrogen at intermediate energies

We present a joint study of the electron capture process in multicharged ions on atomic Hydrogen at intermediate energies. We have applied two methods, the Eikonal Impulse Approximation (EIA) and the hydrogenic-Classical Trajectory Monte Carlo (CTMC) method. Good agreement is found in both total and state-selective electron capture cross sections.

Angular- and state-selective differential cross sections for single-electron capture inp-He collisions at intermediate energies

State-selective electron capture cross sections and projectile scattering-angle differential cross sections for single-electron capture in collisions of protons with He at energies ranging from 50 to 100 keV have been experimentally investigated by means of a reaction microscope. It is shown that for single-electron capture the ground-state transfer is the dominant reaction channel, and excited-state transfer has relative small contributions to the cross sections. The transfer-excitation process has a minor contribution to the total cross sections. As the projectile energy increases, ground-state transfer remains dominant, but the relative contributions of the excited-state transfers become more important. The minor contribution of transfer excitation remains almost unchanged. Furthermore, it was found that for single transfer the present results are in good agreement with independent electron model predictions in the energy regime considered. For transfer excitation, however, there are obvious discrepancies between our measurements and theoretical results. We concluded that the electron-electron-correlation effects may play a role at least in transfer excitation in collisions of protons with He in the energy range considered.

Electron capture and excitation in H+–He (1s2s; 1,3S) collisions

The electron capture and excitation processes in collisions of H+ with excited He*(1s2s; 1,3S) atoms are investigated theoretically by using the two-centre atomic orbital close-coupling method in the energy range 2–200 keV u–1. The interaction of active electrons with the target ion is represented by a model potential different for the singlet and triplet systems of He states. In the electron capture studies, the basis included all the He (1s, n ≤ 6) discrete states and all the H (n ≤ 7) states plus 202 pseudostates, while for the excitation studies all the He (1s, n ≤ 8) states plus 244 pseudostates and all the H (n ≤ 5) discrete states were included in the basis. The excitation cross section results of the present calculations are compared with other theoretical results and good agreement is obtained at high collision energies. For energies below 6 keV u–1, the present excitation and electron capture results are compared with those from the recent quantal molecular orbital close-coupling calculations showing good agreement for the dominant channels. It is found that the cross sections for excitation and state-selective electron capture are significantly different for the singlet and triplet states, particularly in the low-energy region.

State-selective electron capture by O3+ ions on He and H2

Using the cold target recoil ions momentum spectroscopy, we have studied single electron capture in collisions of O3+ ions with He and H2 at 60 keV. The state-selective, angular differential cross sections have been determined by measuring the transverse and longitudinal momentum of recoil ion. The dominant reaction channel is single electron capture into the O2+(2s2p3) and O2+(2s22p3l) states for He and H2 target respectively. The experimental results could be qualitatively understood by reaction windows of the classical over-barrier model.

Electron capture cross section in collision of multi-charged neon ions with ground state hydrogen and helium

The classical trajectory Monte Carlo method and the post form of three-body boundary corrected continuum intermediate state (BCCIS-3B) approximation are employed to calculate the cross sections for total and state-selective electron capture in collision of highly charged Neq+ ions with ground state hydrogen atom in the intermediate to high energy region. In both these methods, the active electron interactions with the partially stripped neon ions are described by a model potential containing both a long-range part and a short-range part. We have also calculated the double electron-capture cross sections in collision of fully stripped neon ion with helium atom in the energy range 80–2,000 keV/amu using the post-form of four-body BCCIS approximation (BCCIS-4B). In BCCIS formalism, the intermediate continuum state of the active electrons with the projectile ion has been taken into account as the projectile charge is greater than the target charge. In addition, state-selective charge transfer cross sections are given in tabular and graphical form.

Electron capture and excitation in slow H+-He(21,3L) collisions

Dynamics of electron capture and excitation processes in collisions of H${}^{+}$ with the excited He(2 ${}^{1,3\phantom{\rule{-0.16em}{0ex}}}{L}_{m}$) atom ($L$ $=$ $S$,$P$) has been studied by using the full quantum-mechanical molecular orbital close-coupling (QMOCC) method with ab initio determined molecular states. 38 singlet and 37 triplet molecular states of the HeH${}^{+}$ system, that asymptotically correlate to the $n$ $=$ 1--4 projectile and target atomic states, have been included in the expansion basis. The shell- and state-selective electron capture and excitation cross sections have been calculated in the 0.01--6 keV/u energy range up to the $n$ $=$ 4 shell and their energy behavior discussed on the basis of identified reaction mechanisms.

State-selective electron capture in He++ H2collisions at intermediate impact energies

State-selective single electron capture in collisions of He${}^{+}$ with H${}_{2}$ molecules is investigated using the cold-target recoil-ion momentum spectroscopy technique. The energy range considered is 6.25 keV/amu (projectile velocity ${v}_{p}$ $=$ 0.5 a.u.) to 50 keV/amu (${v}_{p}$ $=$ 1.41 a.u.). In addition, we explore the transverse recoil momentum distributions that provide direct access to the angular scattering of the receding projectile. The present experimental data are contrasted to the theoretical results obtained by means of the classical trajectory Monte Carlo model with which we find good overall agreement in the energy range explored.

State-selective electron capture in N5+-H and O6+-H collisions

The two-centre atomic orbital close-coupling method is employed to study the electron capture reactions in collisions of N5+ and O6+ ions with ground state atomic hydrogen in the energy range from 0.5 to 200 keV u−1. The interaction of an active electron with the projectile ion is represented by a model potential. Total and state-selective electron capture cross sections for the dominant and subdominant reaction channels are calculated and compared with the available experimental and theoretical data.

State selective electron capture in low energy positron and argon collisions

A three-body classical trajectory Monte Carlo method is applied in the study of positron and argon atom collisions at impact energies between 10 and 60eV. The calculations are based on the independent particle model. We present state selective and total positronium formation cross sections as a function of the impact energy. Our results are compared with recent experimental data.

Dynamics ofO6+ + H electron capture in Debye plasmas and properties of resultingO5+(nl) emission spectra

The electron capture in ${\mathrm{O}}^{6+}$-H collisions is studied by the two-center atomic orbital close-coupling (TC-AOCC) method when the interactions of charged particles are screened and have a Yukawa form. Atomic orbitals and eigenenergies of $n$ $\ensuremath{\leqslant}$ $6$ states on ${\mathrm{O}}^{5+}$ and $n$ $\ensuremath{\leqslant}$ $2$ states on H are calculated as a function of the interaction screening parameter and used in the AOCC dynamics scheme to calculate the electron capture cross sections in the energy range $1$--$200$ keV/u. It is shown that the degree of interaction screening reduces the electron binding energies, the number of open electron capture channels, and the strength of exchange couplings, thus affecting the entire collision dynamics and the magnitude and energy behavior of electron capture cross sections. The state-selective electron capture cross sections in this collision system, as well as the intensities of a number of charge exchange spectral lines, for a number of representative interaction screening strengths are presented and discussed.

Electron capture in C6+ + H collisions in Debye plasmas

The electron capture in C6+–H collisions taking place in a weakly coupled plasma is studied by the two-centre atomic orbital close-coupling method. The interaction between charged particles is represented by the Debye–Hückel potential, appropriate for weakly coupled (Debye) plasmas. The adopted two-centre expansion basis contains all the states with n ⩽ 8 centred on C6+ and all the states with n ⩽ 2 centred on H+. The coupled channel calculations have been performed in the 0.5–100 keV/u energy range in which the results with the adopted basis are convergent. It is shown that the electron capture dynamics in Debye plasmas is strongly affected by the plasma screening of charged particle interactions mainly due to the reduction of electron binding energy and the number of reaction channels when the plasma screening increases. These effects are illustrated on the state-selective electron capture cross sections for a number of representative values of the interaction screening parameter and on the properties of a number of charge exchange spectral lines.

State-selective charge exchange from H(1s), H(n = 2), Li and Na targets by very highly charged ions

In order to provide information for charge exchange spectroscopy diagnostics in new, high temperature tokamak plasmas, n-level state-selective electron capture cross sections are presented for very highly charged ions with charge states q ⩾ 10. Fully stripped ions with charges of 10+, 18+, 26+, 54+ and 92+ are considered in order to provide scaling information over a broad range. The H(1s) and H(n = 2) targets are evaluated at 1, 10 and 100 keV/amu, i.e. plasma core temperatures to those encountered during neutral beam heating. For use in pellet diagnostics, cross sections are given for Li and Na targets at 10 keV/amu. These latter targets also provide a means to experimentally test the accuracy of the H(n = 2) results since they are pseudo-one-electron atoms with ionization energies close to that of excited hydrogen.

State selective electron capture in slow 15N7++ He, Ne collisions studied using cold-target recoil-ion-momentum spectroscopy

Electron capture from He and Ne targets by 15N7+ has been studied at projectile velocities of 0.27 and 0.5 a.u. Cold-target recoil-ion momentum spectroscopy was used to simultaneously measure the Q-value and scattering angle. Projectile velocity and target dependence comparisons of single and double electron capture by the projectile were carried out. Single capture from both targets populates the same excited states with nearly similar angular distributions and relative populations. The populated channels in double capture from He and Ne, however, are quite different. One-step and two-step capture mechanisms are proposed for most of the populated channels in double capture.

State-selective single electron capture studied for C4+ ions on H2 at 40 keV

The experiment on state-selective single electron capture process of C4+ ions with molecular hydrogen at 40keV was performed by reaction microscope. The differential cross sections have been obtained by measuring the longitudinal and transverse momentum of recoil ions. The results show that single electron capture into M shell is by far the dominant reaction channel. The results can be qualitatively understood by reaction window of molecular coulomb barrier model.

Dynamics ofO8++Helectron capture in Debye plasmas

The electron capture in ${\text{O}}^{8+}\ensuremath{-}\text{H}$ collisions is studied by the two-center atomic-orbital close-coupling method when the interactions of charged particles are screened and have a Yukawa form. Atomic orbitals and eigenenergies of $n\ensuremath{\le}7$ states on ${\text{O}}^{7+}$ and $n\ensuremath{\le}2$ states on H are calculated as a function of the interaction screening parameter and used in the atomic-orbital close-coupling dynamics scheme to calculate the electron capture cross sections in the energy range of 0.2--50 keV/u. It is shown that the degree of interaction screening determines the reduction in electron binding energies, the number of open electron capture channels, and the strength of the exchange couplings, thus affecting the entire collision dynamics and the magnitude and energy behavior of state-selective cross sections. The changes in electron binding energies and capture cross sections when the interaction screening varies introduce dramatic changes in the radiation spectrum of ${\text{O}}^{7+}(nl)$ capture states with respect to the unscreened interaction case. The state-selective electron capture cross sections in this collision system, as well as the intensities of a number of charge exchange spectral lines, for a number of representative screening parameter values are presented and discussed.

Experimental differential investigation of state-selective single electron capture in slow He2+-He collisions

State-selective single electron capture in He2+-He collisions at energies ranging from 20 to 40 keV have been studied experimentally by means of cold target recoil ion momentum spectroscopy. The differential cross sections have been obtained by measuring the longitudinal and transverse momenta of recoil ions. The results show that single electron capture into L shell is the dominant reaction channel, which was qualitatively explained with reaction window theory. The measured state-selective cross sections agree with the semi-classical close-coupling calculations, but have some deviations from the results of photon emission measurement. Total angular differential cross sections show that the single electron capture into ground state mainly contributes at small angles, and the single electron capture into excited states contributes at large angles. The oscillating structures of angular differential cross sections are observed, and can be understood by the interference of molecular orbits during the collision processes. The measured angular differential cross sections are compared with other experiments and the quantum-mechanical calculation.

State selective electron capture and excitation in proton collisions with Be

An ab initio study of charge exchange and excitation processes in collisions between protons and beryllium atoms is presented. State selective cross sections are obtained for collision energies between 2.5 × 10−7 and 16 keV/amu, using both quantum and semiclassical treatments. A very large isotopic dependence of the charge exchange cross section is found at energies below 0.6 eV/amu.

Atomic collisions with screened Coulomb interactions: excitation and electron capture in H+ + H collisions

Dynamics of excitation and electron capture processes in the H+ + H(1s) collision system in a Debye plasma is studied as an example of collision systems with screened Coulomb interactions. The two-centre atomic orbital close-coupling method (TC-AOCC) was used with a basis containing all n ⩽ 6 and 7s discrete and 99 quasi-continuum states centred on the target and all n ⩽ 3 discrete states centred on the projectile when describing the excitation process, and vice versa when describing the electron capture. The atomic orbitals and energies of all one-centre states have been determined variationally as a function of screening parameter and used to calculate the state-selective electron capture and excitation cross sections. With an increase of the screening, the bound states successively enter the continuum, thereby reducing the number of reaction channels in the discrete spectrum. The effect of interaction screening on the direct and exchange electronic couplings is also strong. Both these factors significantly affect the dynamics of electron capture and excitation processes. The interaction screening parameter can, thus, be used as a tool for control of the collision dynamics.

Dynamics ofHe2++H(1s)excitation and electron-capture processes in Debye plasmas

Collision dynamics of the ${\mathrm{He}}^{2+}+\mathrm{H}(1s)$ system imbedded in a Debye plasma is studied by the two-center atomic orbital close-coupling (AOCC) method in the energy range $5--300\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{u}$. The atomic orbitals and electron binding energies of atomic states are calculated within Debye-H\"uckel approximation of the screened Coulomb potential and used in AOCC dynamics formalism to calculate the state-selective electron capture and excitation cross sections. The basis contained 174 orbitals centered on the target (all $n\ensuremath{\le}6$ discrete states and 117 quasicontinuum states) and 20 orbitals centered on the projectile (all $n\ensuremath{\le}4$ discrete states). It is demonstrated that the screening of Coulomb interactions in the system progressively reduces the number of available excitation and electron capture channels when the screening parameter increases. The screening of Coulomb interactions introduces changes also in the values of direct and exchange couplings, thus affecting the magnitude and energy behavior of the cross sections. The control of dynamics of collision processes in a Debye plasma by varying the plasma screening of Coulomb interactions in the collision system is discussed.