Cross Sections For Double Capture Research Articles

High-energy two-electron transfer in ion-atom collisions

Two-electron transfer by fast heavy nuclei from heliumlike targets is studied. A detailed sequence of comprehensive computations is carried out in a large keV–MeV range of the projectile energies. This set is illustrated with total cross sections for double capture by alpha particles from helium atoms using several frequently applied four-body quantum-mechanical distorted wave models with the correct boundary conditions. The sensitivity of the obtained total cross sections is examined for different choices of the bound and continuum states. Especially at high energies, the influence of the compactness of the bound states is investigated by reference to the mechanism of the velocity matching kinematic double electron capture. Also considered is the dependence of these cross sections on the electronic screening of the projectile and the target nuclear charges in the bound and continuum states. The impact of this electronic shielding on total cross sections is assessed by reference to the corresponding bare nuclear charges in the bound and continuum states. Relative to all the available experimental data (100–6000 keV), the found striking model-dependence implies that two-electron transfer is sharply different from the associated one-electron transfer involving the same colliding particles.

Influence of electron correlations on double-capture process in proton–helium collisions

The first-order correct-boundary Coulomb–Born distorted-wave approximation is used to study the double-electron capture by protons from the ground-state helium atoms at intermediate and high impact energies. The differential double capture cross sections are obtained as a function of the projectile scattering angle and the total cross sections as a function of the impact energy. In the considered range of impact energy, our calculation shows that although the results are not so sensitive to the static inter-electronic correlations in the initial channel, the strong final-state correlations have a large effect on the magnitudes of the double capture cross sections. The calculated differential and integral cross sections are compared with their available experimental values. The comparison shows a good agreement between the present calculations and the measurements. The comparison of the integral cross sections shows that the present approach is compatible with other theories.

Negative ion productions in high velocity collision between small carbon clusters and Helium atom target

We measured absolute double capture cross section of Cn+ ions (n=1,5) colliding, at 2.3 and 2.6 a.u velocities, with an Helium target atom and the branching ratios of fragmentation of the so formed electronically excited anions Cn−*. We also measured absolute cross section for the electronic attachment on neutral Cn clusters colliding at same velocities with He atom. This is to our knowledge the first measurement of neutral-neutral charge exchange in high velocity collision.

Double charge transfer cross sections in collision of protons with helium atom

Theoretical total cross sections for the double charge transfer in proton-helium collisions are analysed using the four-body boundary corrected continuum intermediate state approximation in the energy range of 30 to 500 keV. We have also obtained differential double capture cross sections as a functions of projectile scattering angle. The intermediate continuum state of each of the active electron has been taken into account in this formalism. Our calculated results are compared with the previous experimental and theoretical values. The agreement is very encouraging.

Differential double capture cross sections inp+Hecollisions

We have measured differential double capture cross sections for 15 to $150\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ $p+\mathrm{He}$ collisions. We also analyzed differential double to single capture ratios, where we find pronounced peak structures. An explanation of these structures probably requires a quantum-mechanical description of elastic scattering between the projectile and the target nucleus. Strong final-state correlations have a large effect on the magnitude of the double capture cross sections.

Simplified Strategies to Study Complex Ion–H2 Collisions

Single and double capture cross sections in collisions between Li+ and N5+ ions with H2 molecules are calculated in the range of impact velocities v = 0.05–0.5 a.u., using the Franck-Condon approximation and a molecular close coupling expansion. Given the complex characteristics of these systems, application of ab-initio methods becomes prohibitive, and approximate techniques have been implemented.

Scaling laws for single and double electron capture inAq++Hecollisions(q>~ZA−2)at low impact velocities

We present empirical scaling laws, as a function of the projectile charge state, for single and double electron capture in slow collisions between highly charged ions and He atoms at impact velocities of 0.1 and 0.5 a.u. The fitting parameters are shown to be suitable for predicting the populated states in single and double electron capture. The scaling law for single capture is found to be nearly independent of the projectile velocity in the range 0.1--0.5 a.u. The same fitting procedure is followed for double electron capture at the velocity of 0.5 a.u. since independent monoelectronic transitions, due to electron-nucleus interactions, are dominant. At this velocity, the scaling law for the projectile charge dependence of double electron capture cross sections is found to be similar to that for single electron capture. At the lower velocity of 0.1 a.u., where dielectronic processes caused by the electron-electron interaction gain importance, the charge dependence of double capture cross sections is strongly modified.

State-selective differential cross sections for double-electron capture in 0.25–0.75-MeVHe2+-He collisions

For 0.25{endash}0.75-MeV He{sup 2+} on He collisions we have measured total state selective double capture cross sections and cross sections differential in projectile scattering angle. For 0.25 MeV we present also state-selective scattering-angle-dependent double-capture cross sections. The projectile energy loss (the final electronic state) as well as the transverse momentum transfer (i.e., the projectile scattering angle) have been obtained by measuring the momentum vector of the recoil ion using cold target recoil ion momenum spectroscopy. The resonant transfer to the ground state is found to be by far the dominant double-capture channel. Capture to nonautoionizing excited states is smaller by about a factor of 7, and results in larger scattering angles than the ground-state double capture. {copyright} {ital 1998} {ital The American Physical Society}

Study of two electron capture in highly charged ion-atom collisions by Rydberg spectroscopy

We present an extensive study of double electron capture processes in slow collisions of bare ions from C6+ to Al13+ with gases from He to Xe and metallic vapour (strontium, barium, zinc) targets. The Stabilized Double Capture (S.D.C.) process, in which two electrons are captured and stabilized on the projectile, has been studied by measuring the radiative stabilization ratio R2s(R2s = σS.D.C./(σS.D.C. + σA.D.C.); σS.D.C. and σA.D.C. stand for the S.D.C. and Autoionized Double Capture cross sections) and by using optical spectroscopy of Rydberg transitions. We found that the population of asymmetrical configurations (n, n' » n) is responsible for high radiative stabilization ratios. The population of these configurations is due to Auto-transfer into Rydberg state (A.T.R.) and Transfer excitation (T.E.) processes. In the case of F9+ + Ne collision system, the n' distribution of excited Rydberg states has been deduced from a precise analysis of the light emitted by Rydberg transitions. The distribution is compared with a theoretical distribution issued from the A.T.R. model. We also measured the stabilization ratio for higher charge state ions Ar14+, Ar16+, Ar17+, Kr18+, Xe25+, Xe27+ on strontium and rare gas collisions and observed a slight enhancement of R2s values for higher charge states. The observation of high Rydberg transitions for Xe27+ on Xe collision shows that the radiative stabilization process is also explained by a population of asymmetrical configuration issued from A.T.R. and T.E. processes.

Observation of sequential electron capture to individual highly-charged Th ions

Electron transfer from H2 to Th79+ was studied in a cryogenic Penning trap by non-destructively observing the sequential development with time of the charge state of one or a few stored particles. The ratio of the true double capture cross section to the total capture cross section was found to be 0.21 ± 0.11 at a mean center-of-mass energy near 6 eV.

Two-electron capture by He2+, Li3+, and B5+ in the independent-event model.

We apply a single-channel distorted-wave approximation in conjunction with the independent-event model to calculate the double-capture cross sections by ${\mathrm{He}}^{2+}$, ${\mathrm{Li}}^{3+}$, and ${\mathrm{B}}^{5+}$ from neutral He. The capture probabilities of the two electrons in two successive independent events are calculated by a single-channel distorted-wave approximation with realistic screening charge parameters. Our T matrix has been evaluated analytically and therefore the present method is much simpler than other available rigorous calculations. Our results are compared with the measurements and other calculations. It is found that the present method gives fairly good agreement with the measurements. \textcopyright{} 1996 The American Physical Society.

Measurement of Charge Exchange between H2 and Low-Energy Ions with Charge States 35

Charge-exchange cross sections for collisions between ${\mathrm{H}}_{2}$ and ions with unusually low center-of-mass energies (\ensuremath{\approx}6 eV) and high-charge states (${\mathrm{Xe}}^{q+}:35, 43\ensuremath{\le}q\ensuremath{\le}46$, ${\mathrm{Th}}^{q+}:73\ensuremath{\le}q\ensuremath{\le}80$) have been measured using a Penning ion trap. A nondestructive measurement technique permitted both primary and product ion analysis. The data, averaged over the charge states indicated above, are consistent with a total cross section that scales linearly with ion charge $q$. The true double-capture cross section is found to be about 25% of the total cross section at high $q$. The results are compared to predictions of the absorbing-sphere model.

Competition between radiative and non-radiative decay for doubly excited 2lnl' and 3lnl' states in C4+

Using the truncated diagonalization method with a second-order energy correction in a B-spline basis, energies, autoionization and radiative rates are obtained for 2lnl' (n<or=7) and 3lnl' (n<or=5) singlet terms in the doubly excited He-like carbon ion and compared with experimental as well as other theoretical results where available. Branching ratios for radiative decay are determined in order to estimate the importance of radiative decay in the interpretation of ejected-electron spectra. For a proper determination of cross sections for resonant transfer excitation or double capture into 2lnl' states, it is found that it is necessary to take radiative decay into account. For the determination of cross sections for double capture into 3lnl' levels, the probability of radiative decay cannot be neglected entirely, although it is much less significant than for 2lnl'. For the latter configuration radiative decay is the dominant decay mode for most terms with n equal to or larger than 6.

One- and two-electron capture in collisions of slow B4+ and Be4+ ions with helium.

One-electron and two-electron capture in slow ${\mathrm{B}}^{4+}$-He and ${\mathrm{Be}}^{4+}$-He collisions are studied within the semiclassical close-coupling description with two-electron atomic basis sets. For ${\mathrm{B}}^{4+}$-He collisions, we find large two-electron capture cross sections to 2l2l' doubly excited projectile states. Double-capture cross sections to 2${\mathit{s}}^{2}$ $^{1}$${\mathit{S}}^{\mathit{e}}$, 2s2p $^{1}$${\mathit{P}}^{\mathit{o}}$, and 2${\mathit{p}}^{2}$ $^{1}$${\mathit{D}}^{\mathit{e}}$ states and the relative M distributions are found to be in good agreement with recent experiments. Earlier data on ${\mathrm{B}}^{3+}$ production are found to contain large portions of double transfer followed by autoionization. For the ${\mathrm{Be}}^{4+}$-He system, the summed cross sections from this work agree with an earlier theoretical study within the adiabatic model. Discrepancies remain, however, for this system in the distribution of double transfer over final states.

State-selective double capture in collisions of bare ions with helium atoms at low energies. I. Total cross sections

Double capture cross sections to individual doubly excited states for collisions between bare ions of carbon and oxygen with helium atoms in the energy range of a few keV amu-1 are calculated. By adopting an independent-electron approximation, the capture probability to each doubly excited state is calculated in terms of products of single capture amplitudes properly weighted with configuration-interaction coefficients, while the single capture amplitudes are calculated using a two-centre atomic orbital close-coupling expansion method with an extended basis set. The resulting cross sections are compared with recent results from high-resolution Auger electron measurements to assess the validity of the independent-electron approximation as well as the mechanism of double capture. It is shown that there is qualitative agreement between the present results and experiments.

Fully converged time-dependent Hartree-Fock results for He2+-He: Correlation in inclusive charge transfer.

Previous time-dependent Hartree-Fock (TDHF) calculations of collisions between He ions and He atoms have been extended with improved accuracy. New features include using a Hermitian Coriolis term and installing classical quantum coupling, which allows completely self-consistent treatment of both the nuclear and electronic coordinates. Increased computer capabilities have allowed us to eliminate previous numerical limitations. We have calculated the single- and double-capture cross sections in the energy range of 1 to 250 keV. Comparison with experiments allows us to investigate the strengths and weaknesses of TDHF theory. Since correlation is best defined to be that which is not included in single-particle descriptions, we discuss converged results with regard to what they reveal about correlation.

Double capture in the O8++He collision investigated by electron spectroscopy (80 keV, 10')

Double capture into (3, n) autoionising states of O6+ is observed in the O8++He collision by electron spectroscopy at 80 keV, 10'. A comparison with the authors' theoretical positions and lifetimes allows to identify (3, 3) states as well as three groups of (3, 4) states; some theoretical (3, 5) values are also given which agree with the corresponding experimental line position. By a fitting procedure of the (3, 3) part of the electron spectrum, relative weights of (3, 3) states are determined and it is found that the (3, 3)1Ge state accounts for 42% of the double capture into the (3, 3) configuration; also 24% of the double capture into the (3, 4) configuration is found to occur into the highest angular momentum states 1Ho(1) and 1Ge(2). A comparison of the present experimental findings with earlier data the authors have obtained for the N7++He and Ne8+(1s2)+He systems, is made. The influence of the core of the multicharged ion is studied by comparing O8+ and Ne8+(1s2) results; it is found that only the L distributions differ from each other, the n distribution and total cross section being the same. The total double capture cross section is found to be large and it is compared with available experimental and theoretical data.

Double- and single-electron capture and loss in collisions of 1-2-MeV/u boron, oxygen, and silicon projectiles with helium atoms.

Single- and double-electron-capture cross sections were measured for 1--2-MeV/u bare ${\mathrm{B}}^{5+}$, ${\mathrm{O}}^{8+}$, and ${\mathrm{Si}}^{14+}$ projectiles in collisions with helium atoms. The measured double-electron-capture cross sections are 2--3 orders of magnitude smaller than corresponding single-capture cross sections. The single-capture cross sections are well described by continuum-distorted-wave calculations. For ${\mathrm{B}}^{5+}$, the double-capture cross sections at high velocities are in reasonable agreement with scaled Oppenheimer-Brinkman-Kramers calculations, assuming capture of independent electrons. Cross sections for electron capture and loss are also reported for nonbare ions (${\mathrm{O}}^{6+}$, ${\mathrm{O}}^{7+}$, ${\mathrm{Si}}^{8+}$, and ${\mathrm{Si}}^{13+}$). Single-capture cross sections for nonbare ions are about 30% smaller than corresponding cross sections for bare ions with the same ionic charge q. The reported electron-loss cross sections are well described as ionization of an ion by an incident helium nucleus and two free electrons, using theoretical ionization cross sections in the Born approximation.

Charge transfer and ionization of lithium by protons and helium ions.

Proton and helium-ion impact on atomic lithium is studied for impact energies ranging from 15 to 200 keV. By observing the final lithium charge states, cross sections for pure single electron capture from lithium as well as for single capture plus ionization are presented. Cross sections for double electron capture by ${\mathrm{He}}^{2+}$ projectiles are also measured and the decrease in the double-capture cross section below 20 keV/amu is confirmed. Cross sections for ionization of lithium by these same projectiles are also presented and compared with theoretical calculations.

DoubleK-shell electron capture for ion-atom collisions at intermediate energies

A three-state, two-center atomic expansion method is used to investigate double-electron capture from the $K$ shell of multielectron atoms to the $K$ shell of projectiles in the energy region where the projectile velocity is nearly equal to the orbital velocity of the target $K$-shell electron. By adopting an independent-electron approximation, the Coulomb interaction between the two electrons is replaced by monopole terms with a screening charge. The validity of this approximation is checked by comparing the cross sections for double capture in $p$-He and ${\mathrm{He}}^{++}$-He collisions obtained from this method with other theoretical calculations and experiments. Double-electron-capture cross sections from the $K$ shell of Ne atoms by ${\mathrm{F}}^{9+}$, ${\mathrm{O}}^{8+}$, and ${\mathrm{N}}^{7+}$ ions are then calculated; the results are shown to compare well with the neon Auger hypersatellite intensities measured by Woods et al.