Total Electron Capture Cross Sections Research Articles

State selective electron capture by slow S2+ recoil ions in atomic and molecular hydrogen

Translational energy spectroscopy has been used to study state selective electron capture by slow S2+ ions from a recoil ion source. These ions are important in some astrophysical plasmas. Measurements in both atomic and molecular hydrogen have been carried out in the impact energy range 2-8 keV. The main excited product channels have been identified and their relative importance has been assessed. In the case of S2+-H collisions, the authors' observation of strong exothermic and endothermic channels is at variance with the very small total electron capture cross sections predicted by Landau-Zener calculations.

Orientation-dependent atomic model for electron transfer in ion-molecule collisions: Applications to H++H2 and He2++H2.

Single-electron-capture cross sections in collisions of protons and /alpha/ particles with ground-state H/sub 2/ molecules have been studied for projectile energies in the range of 1--500 keV/amu. By treating the ground-state H/sub 2/ wave function as a linear combination of atomic orbitals from the two atoms, the electron-capture amplitude in ion-molecule collisions is expressed as the coherent superposition of two single-electron-capture amplitudes in /ital ion/-/ital atom/ collisions with the relative phase between the two amplitudes dependent on the collision velocity and the orientation of the molecule. The dependence of the capture cross sections with respect to the molecular orientation was examined. Total electron-capture cross sections averaged over the molecular orientations were obtained and found to be in good agreement with experiments.

Molecular treatment of single-electron capture in Li3++Li collisions

The single charge exchange process in Li3++Li(2s) collisions is investigated in the impact energy range 2.25*10-4-1 keV amu-1. The adiabatic potential energies and radial coupling matrix elements of the Li23+ molecule are computed in a one-electron approximation, using a model potential method. Those are subsequently employed to solve the collision dynamics by a full quantum mechanical treatment or by using the semiclassical impact parameter approach. The results for the total electron-capture cross section present a high and broad maximum value around 220 AA2 at 0.1 keV amu-1. A disagreement of more than one order of magnitude with the theoretical results reported by Stollberg and Hai-Wong Lee (1984) is discussed.

Determination of differential cross sections in classical trajectory Monte Carlo calculations

Classical three-body calculations have been performed to predict total and doubly differential ionisation probabilities and cross sections for 10 keV to 100 MeV H++H and 100 keV H++He collisions. Total and differential electron capture cross sections have been calculated from 3 keV to 1 MeV for H++H. The results are in good agreement with measured cross sections and with other theoretical results known to be relatively good approximations in specific impact energy regimes. The validity of the classical approximation especially at high projectile velocities is discussed.

K- and l-shell resonant transfer and excitation in ion-atom collisions

Recent experimental studies of resonant transfer and excitation (RTE) in ion-atom collisions are reviewed. In the RTE process correlated electron capture and projectile excitation occur together in a single encounter with a target atom. Measurements of Ca q+ + H 2 ( q= 10–19) from 100 to 370 MeV establish the projectile charge-state dependence of K-shell RTE and provide a detailed test of the theory. Structure due to RTE is observed in the energy dependence of the total electron-capture cross sections for this collision system. A comparison of the Ca 17++H 2 data with previous results for Ca 17++He demonstrates the effect of the target-electron momentum distribution on the RTE process. Studies of 230–610 MeV Nb 31+ +H 2 provide information about RTE involving projectile L-shell excitation. All the measurements are in reasonable agreement with theoretical calculations.

Transfer excitation in low-energy (keV amu-1) multiply charged ion-atom collisions

Evidence for simultaneous electron excitation and capture is found in collisions between Ne7+(1s22s) ions and He atoms. Population of the Ne6+(1s22p3l) states is found to amount to up to 16% of the total electron capture cross section. Unambiguous identification of the transfer excitation channel has been obtained with a new technique where charge states of both projectile and target as well as energy gain and scattering angle are all simultaneously determined. The relative importance of 'correlated' electron processes is discussed in view of the presently available data.

Subshell-selective electron capture in collisions of C4+, N5+, O6+with H, H2and He

By means of optical spectroscopy the authors have determined absolute subshell-selective single electron capture cross sections sigma nl for collisions of C4+, N5+ and O6+ with atomic and molecular hydrogen and helium at impact velocities between 0.1 and 0.5 au. The absolute sensitivity calibration of the monochromators is discussed briefly. In addition they have measured the total single electron capture cross section sigma t by means of charge state analysis, for atomic and molecular hydrogen as targets. In all cases it is found that one or two n shells are dominantly populated and that the total cross section is only weakly dependent on the impact velocity, in contrast to the subshell-selective cross sections sigma nl. The main features of the results obtained are qualitatively explained by means of simple diabatic potential diagrams and the classical overbarrier transition model. For several systems a detailed comparison can be made with recent theoretical calculations, showing satisfactory agreement.

Inelastic H+-Li(2s) collisions (2-20 keV). III. Electron capture into the H(2s) subshell

For pt.II, see ibid., vol.17, p.4201 (1984b). Measured absolute cross sections for electron capture into the H(2s) subshell in H+-Li(2s) collisions at impact energies from 2 to 20 keV are presented and compared with available theoretical results, showing agreement within experimental errors. When summing up these and earlier measured cross sections for electron capture into H(2p) and H(3,l) subshells, consistency with directly determined total electron capture cross sections is found.

Observation of low-energy Z oscillations in total electron-capture cross sections for bare projectiles colliding with H and H2.

An oscillatory charge dependence of total electron-capture cross sections has been observed for fully stripped projectiles having charge $6<~Z<~10$ incident on H and ${\mathrm{H}}_{2}$ at collision energies below 3.0 keV/amu. The oscillations are attributed to the discreteness of the projectile final-state spectrum and the strong selectivity of the capture process at a given energy. Relative maxima in the cross sections occur for odd projectile charge in the case of H, and for even projectile charge in the case of the ${\mathrm{H}}_{2}$ target. This shift is explained in terms of a Landau-Zener curve-crossing picture of the electron-transfer collision.

Electron capture by slow Alq+ ions colliding with hydrogen.

Total electron-capture cross sections have been measured for collisions of ${\mathrm{Al}}^{\mathrm{q}+}$ (q=2--10) with H and ${\mathrm{H}}_{2}$ in the 20--120-eV/amu energy range. The general trend of the cross sections for both H and ${\mathrm{H}}_{2}$ is an approximately linear increase with ionic charge state. The ${\mathrm{Al}}^{2+}$+H and ${\mathrm{Al}}^{3+}$+H systems were investigated theoretically using the molecular-orbital method with a pseudopotential to represent the ${\mathrm{Al}}^{3+}$ ion core. Coupled-channel calculations realize good agreement with experiment. An anomalously small cross section for ${\mathrm{Al}}^{2+}$+H is attributed to the fact that of the $^{1}\mathrm{\ensuremath{\Sigma}}$ and $^{3}\mathrm{\ensuremath{\Sigma}}$ entrance channels, only the $^{1}\mathrm{\ensuremath{\Sigma}}$ molecular state contributes to the cross section.

Total and state-selective capture by slow N2+ions in atomic and molecular hydrogen

Translational energy spectroscopy of a tungsten tube furnace has been used to identify the main collision channels for electron capture by N2+ ions in atomic hydrogen. The results have been used in conjunction with total electron capture cross sections measured in a parallel experiment to determine cross sections for capture into the 3Do, 3Po, 1Do and 3P states of N+ in the range 0.6-8 keV. The observations are shown to be consistent with recent theoretical predictions by Dalgarno et al. (1984) for the 3Do product channel. Total cross sections for one-electron capture by N2+ in both H and H2 have been determined in the range 0.6-200 keV and a simple beam attenuation technique has been used to investigate the influence of metastable N2+ (4P) ions in the primary beam.

Etude spectroscopique de la collision d'échange de charge entre Ne 8+ et He

The charge exchange collision between Ne 8+ and He has been studied by VUV spectroscopy. Emission cross sections of the observed transition have been measured. Absolute excitation cross sections and the total single electron capture cross section have been deduced Etude spectroscopique (160-900 A) pour des collisions a 48, 64 et 80 keV. Mesure des sections efficaces d'emission. Deduction des valeurs absolues des sections efficaces d'excitation des niveaux n=4 de Ne VIII et de la section efficace totale d'echange de charge

Computation of total cross-sections for electron capture in high energy collisions. III

Programmes CDW1 et CDW2 pour calculer les sections efficaces totales de capture electronique lors de collisions atome-ion a haute energie, a l'approximation de l'onde deformee pour le continuum

Electron capture by C4+, N5+and O6+from atomic hydrogen in the keV amu-1energy range

Total electron capture cross sections by C4+(1s2), N5+(1s2) and O6+(1s2) from atomic hydrogen are calculated in the molecular approximation. The problem is treated in a one-electron approximation by using a model potential approach. Electronic momentum transfer is accounted for through the use of the common electron translation factor proposed by Errea et al. (1982). Comparison is made with experiment in the energy range 0.25-25 keV amu-1.

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

Computation of total cross-sections for electron capture in high energy collisions. II

The collision of target electrons with ionic projectiles, in highly asymmetric ion-atom collisions, can be simulated within the impulse approximation (IA) by a beam of electrons scattering off the projectile ion, with a collision energy broadened by the momentum distribution (Compton profile) of the target. This description works particularly well for 180° elastic scattering of target electrons off the ion (Phys. Rev. A 41 (1990) 4816 [1]) appearing in the laboratory as the well known binary encounter electron peak. Inelastic processes due to target-electron-projectile-electron interactions (e-e) such as e-e excitation (Phys. Rev. Lett. 62 (1989) 2261 [2]) and e-e ionization (Phys. Rev. Lett. 63 (1990) 1938; J. Phys. B 24 (1991) 977; Phys. Rev. A 46 (1992) 1374; Phys. Rev. Lett. 69 (1992) 3033, 72 (1994) 3170, 72 (1994) 3166 [3–8]) are also well documented. However, for these processes, the target electron undergoing inelastic scattering has never been explicitly identified and measured in ion-atom collisions. A simple theoretical Born-IA treatment gives the expected double differential cross sections and their angular distributions for H-like ions in collision with H2 targets. The viability of observing these effects in ion-atom collisions by electron spectroscopy is investigated.

Computation of total cross-sections for electron capture in high energy collisions. II

We report on theoretical total cross sections for electron capture from the ground state of atomic hydrogen H(1s) by fast multiply-charged nuclei H+, He2+, Li3+, Be4+, B5+, C6+, N7+, O8+ and F9+. The prior form of the boundary-corrected continuum intermediate states (BCIS) method is used. For a single fixed initial ground-state (i=1s), comprehensive computations are carried out for a sequence of the transitions to the final state (n,l,m) with 1≤n≤nmax, including all the energy degenerate sub-levels (−l≤m≤l,0≤l≤n−1). The maximal quantum number nmax is respectively set to 4 for H+, He2+ and Li3+, to 5 for Be4+, to 6 for B5+, as well as to 7 for C6+, N7+, O8+ and F9+. The reported nine tables provide these state-selective cross sections (Qif) and their state-summed counterparts (QΣ). The cross sections QΣ for the sum over all the final states are evaluated using the exact results of Qif for n≤nmax and the scaled cross sections for n>nmax. The scaling is based on the Oppenheimer n−3 rule. In addition to these tables, at the same impact energies 20–3000 keV/amu, the cross sections from the BCIS method are presented graphically in eight figures for the nuclei Li3+,Be4+,B5+,C6+,N7+,O8+ and F9+. This complements our recently published cross sections for electron capture by protons (H+) and alpha particles (He2+) from H(1s). In the present figures, whenever available, the corresponding experimental data are compared with our results for QΣ. Also included in these comparisons are the corresponding results for QΣ from the continuum distorted wave (CDW) method. The cross sections tabulated here can be used in versatile applications ranging from plasma physics and astrophysics through fusion research to ion therapy.

Electron capture by slowFeq+ions from hydrogen atoms and molecules

Total electron-capture cross sections have been measured for collisions of ${\mathrm{Fe}}^{q+}$ ions ($3\ensuremath{\le}q\ensuremath{\le}14$) with H and ${\mathrm{H}}_{2}$ at energies in the 10-95 eV/amu range. The cross sections increase approximately linearly with increasing ionic charge $q$, and, for atomic hydrogen, can be represented by the empirical formula: $\ensuremath{\sigma}=q\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}15}$ ${\mathrm{cm}}^{2}$ with an rms deviation of \ifmmode\pm\else\textpm\fi{}26%. For ${\mathrm{Fe}}^{5+}$ and ${\mathrm{Fe}}^{6+}$, for which higher-energy data exist, the cross sections are found to be essentially energy independent over the range $17lEl2000$ eV/amu. The cross sections for an atomic H target average 26% larger than for ${\mathrm{H}}_{2}$. Agreement with generalized theoretical models is within a factor of 2 or better.

Population of the 3s State in Hydrogen Due to H+, H+2 and H+3 Impact on Ethylene, 1-Butene and Cis-2-Butene

Absolute cross sections were measured for capture or dissociation into the 3s state of atomic hydrogen due to impact of 14-28 keV H+, H+2 and H+3 ions on ethylene, 1-butene and cis-2-butene. The cross sections obtained for the butene targets were almost identical and support the additive rule for cross sections. Ethylene cross sections were 21% lower than the values predicted by the additive rule, which agrees well with the observed trend toward reduction in total electron capture cross sections for protons on these gases at much higher energies.

Angular scattering in slow multiple-charged ion, atom collisions

The C6++H system is used to illustrate the importance of large-angle scattering in collisions between slow multiply-charged ions and atoms. A quantum mechanical description based on a diabatic formalism of the collision system is used to obtain differential cross sections for electron capture in the relative velocity range nu =1*107 to 3*107 cm s-1 (Ecm approximately=48 to 430 eV). The threshold for angular scattering is at E theta approximately=0.2 keV deg which correlates with the curve crossing between initial and final molecular states located at Rx approximately=8a0. The centre-of-mass acceptance angles required to observe fixed fractions of the total electron capture cross sections are presented. Representative examples for the detection of 90% of the total electron capture cross section requires angular acceptances of 78 degrees at 1*107 cm s-1 and 8.5 degrees at 3*107 cm s-1.