Electron Capture Cross Research Articles

Hyperspherical close-coupling calculations for electron-capture cross sections in low-energyNe10++H(1s)collisions

We present total and partial electron-capture cross sections for ${\mathrm{Ne}}^{10+}+\mathrm{H}(1s)$ collisions at energies from $0.01\phantom{\rule{0.3em}{0ex}}\mathrm{eV}\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ using the hyperspherical close-coupling method. Good agreements with the previous calculations by the classical-trajectory Monte-Carlo method are found for total capture cross section, but not for partial cross sections, especially below about $200\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}$. We found that the total cross section is mainly due to the population of $n=7$ channels and only at energies above $50\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}$ $n=5,6$ channels begin to contribute to the total cross section.

Single Electron-Capture Cross Sections in C5+,6++C3+, N6+,7++N4+, and O7+,8++O5+ Collisions

Single electron-capture cross sections in collisions of bare and hydrogen-like ions with lithium-like ions of C, N, and O are calculated in the collision energy range between 0.05 and 16 keV/amu using a molecular-state close-coupling method. For the lithium-like ions, both the ground and first excited states are considered. The cross sections are calculated for capture into n =2 and 3 states of the bare and hydrogen-like ions and for excitation and de-excitation of the lithium-like ions. None of the electron-capture cross sections are larger than 10 -15 cm 2 in the collision energy range considered in the present study. At lower collision energies, the cross sections decrease due to Coulomb repulsion between the ions. Partial cross sections for capture into 1s3s, 1s3p, and 1s3d levels for the hydrogen-like ion collisions are on the same order of magnitude because the final states are mixed with each other by the electric field induced by the helium-like ions.

Elastic and inelastic processes inH++NH2collisions between 0.5 and1.5keV

Electron capture and direct elastic scattering in collisions of ${\mathrm{H}}^{+}$ ions with $\mathrm{N}{\mathrm{H}}_{2}$ molecules between 0.5 and $1.5\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ are theoretically investigated. A molecular representation is adopted within a fully quantum-mechanical approach. We consider the following four different molecular configurations for collision dynamics: (I) the proton approaches the N atom along the bisector of the $\mathrm{H}\mathrm{N}\mathrm{H}$ bond angle, passing the midpoint of the $\mathrm{H}\mathrm{H}$ line, (II) it comes along the same line as in (I), but in the opposite direction, (III) the proton approaches the N atom in the $\mathrm{H}\mathrm{N}\mathrm{H}$ plane and perpendicularly to the bisector of the $\mathrm{H}\mathrm{N}\mathrm{H}$ bond angle, and (IV) the proton approaches the N atom perpendicularly to the $\mathrm{H}\mathrm{N}\mathrm{H}$ plane. Differential cross sections for elastic scattering and electron capture are calculated at 1.5 and $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$ for these different molecular orientations. Total cross sections for these four orientations as well as orientation-averaged total cross sections are also calculated. Our results indicate that electron-capture dynamics and corresponding electron-capture cross sections depend substantially on the molecular configuration, thus revealing a strong steric effect.

Identification of sulfur double donors in 4H-, 6H-, and 3C-silicon carbide

Sulfur ions are implanted into 6H-, 4H-, and 3C-SiC. Admittance and double correlated deep level transient spectroscopy investigations reveal that S atoms form double donors in SiC, which reside on lattice sites. The number of double donors observed corresponds to the number of inequivalent lattice sites of the particular SiC polytype. The temperature dependence of the electron capture cross section is determined by the filling pulse method. It is shown that the capture of electrons into S donors is dominated by the cascade mechanism.

Asymptotic properties of the three-Coulomb-center problemeZ1ZZ

An analytic study is presented of the asymptotic properties of the three-Coulomb-center problem $e{Z}_{1}ZZ$. The electron energies and wave functions of this system, where $e$ designates an electron and $Z$, ${Z}_{1}$ are bare nuclei, are calculated asymptotically exactly for large distances $L$ between the fragments of a quasimolecular system. The electron exchange interaction between $eZZ$ and ${Z}_{1}$ fragments is also calculated asymptotically exactly and used to estimate the electron-capture cross section in slow collisions of $Z=1$ and ${Z}_{1}=2,3,4$ systems.

Updated single-electron-capture cross-section scaling rule forAq++H2collisions

An updated high-energy universal scaling rule for single electron capture cross sections is presented for A{sup q+}+H{sub 2} collisions. The current scaling rule is built empirically on experimental data using a wide range of projectile ions and collision energies. The inclusion of such a broad range of values results in a model that more accurately predicts the single capture cross sections for a much wider range of collision systems. The current scaling rule is compared to three similar scaling rules to illustrate how using limited data to develop a universal scaling rule effects a models general predictive power. The effects of all four scaling rules on both experimental and theoretical data are examined.

Anisotropic plasma effects on electron capture process in an anisotropic plasma

The anisotropic and plasma screening effects on the electron capture process are investigated in an anisotropic plasma in the presence of an external electric field. The Bohr–Lindhard model with the impact parameter analysis is applied to obtain the electron capture probability in the anisotropic plasma as a function of the impact parameter, Debye length, and collision energy. The result shows that the electron capture cross section decreases with increasing collision energy. It is also found that the anisotropic effect strongly suppresses the electron capture cross section in the anisotropic plasma.

X-ray emission cross sections following charge exchange by multiply charged ions of astrophysical interest

State selective $nl$-electron capture cross sections are presented for highly charged ions with $Z=6--10$ colliding with atoms and molecules. The energy range investigated was from $1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}(v=0.006\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.})\text{to}\phantom{\rule{0.3em}{0ex}}100\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu}(v=2.0\phantom{\rule{0.3em}{0ex}}\mathrm{a.u.})$. The energy dependence of the $l$-level populations is investigated. The $K$ shell x-ray emission cross sections are determined by using the calculated state-selective electron capture results as input and then applying hydrogenic branching and cascading values for the photon emission. A major shift in the line emission from being almost solely Lyman-$\ensuremath{\alpha}$ transitions at the highest collisions energies to strong high-$n$ to $1s$ transitions at the lowest energies is observed. The calculated cross sections are in reasonable accord with measurements made by Greenwood et al. [Phys. Rev. A 63, 062707 (2001)], using ${\mathrm{O}}^{8+}$ and ${\mathrm{Ne}}^{10+}$ on various targets at $3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{amu}$. The calculations are also in accord with x-ray emission cross section data obtained on the EBIT machine at Lawrence Livermore National Laboratory (LLNL) where ${\mathrm{O}}^{8+}$ and ${\mathrm{Ne}}^{10+}$ high resolution measurements were made at a temperature of $10\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{amu}$ for a series of targets with varying ionization potentials. The ${\mathrm{Ne}}^{10+}$ data clearly shows the contribution from multiple capture followed by Auger autoionization in the line emission spectra. Our calculated line emission cross sections are used to provide an ab initio determination of the soft x-ray spectrum of comet C/Linear 1999 S4 that was observed on the Chandra X-ray Observatory. The calculations show that the spectrum is due to the charge exchange of the neutral gases in the comet's coma with the ions of the slow solar wind.

State-Selective Electron Capture for keV He2+ Ions onHelium Collisions Studied by Recoil Momentum Spectroscopy

State-selective single electron capture cross sections are measured byrecoil ion momentum spectroscopy technique for He2+ on He at30 keV incident energy. The cross sections for capture into ground andexcited states are obtained and compared to classical modelcalculations as well as to the quantum mechanical calculations. Theexperimental results are in good agreement with quantum mechanicalresults.

Calculation of Rate Coefficients for Electron Capture in Collisions of O2+and N2+Ions with H

We present calculations of electron capture cross sections in collisions of O2+ and N2+ with H(1s) for impact energies 0.001 eV < E < 10 keV and the corresponding rate coefficients for temperatures 102 K < T < 105 K. Our molecular close-coupling treatment leads to significant differences from the capture rates usually employed in the modeling of astrophysical plasmas.

Electronic properties of interstitial iron and iron-boron pairs determined by means of advanced lifetime spectroscopy

As dissolved iron is one of the most common lifetime-killing contaminants in silicon, its coexisting defect configurations, interstitial iron (Fei) and iron-boron pairs (FeB), are investigated on an intentionally iron-contaminated silicon sample by means of temperature-dependent lifetime spectroscopy (TDLS) and injection-dependent lifetime spectroscopy (IDLS). In good agreement with the literature, the study identifies the known Fei donor level at Et−EV=(0.394±0.005)eV and determines its symmetry factor as k=σn∕σp=51±5, which is an order of magnitude lower than expected from the literature. Using the well-confirmed k factor, the poorly confirmed electron-capture cross section is redetermined as σn=(3.6±0.4)×10−15cm2 at 300K. In addition, the observed exponential σ(T) dependence identifies the multiphonon emission mechanism as the dominant capture mechanism for electrons with an activation energy E∞=0.024eV. Concerning the defect related to the iron-boron pair, the study unambiguously identifies the deep FeB acceptor level as the dominant FeB recombination center. While the known average value EC−Et=(0.26±0.03)eV for the energy level is well confirmed, the symmetry factor is determined as k=0.45, which represents an intermediate value among the scattered results from the literature. Additional spectroscopic information from the defect transformation allows both capture cross sections of the FeB defect to be determined (σn=2.5×10−15cm2 and σp=5.5×10−15cm2). Being suggested as a fingerprint of iron in the literature, the crossover position of the Fei- and FeB-dominated IDLS curves (at 300K) is experimentally proved to be doping dependent which is accurately predicted on the basis of the extracted set of Fei and FeB defect parameters. An additional fingerprint of iron is found by the qualitative change of the TDLS curve upon illumination and its S-like shape under dark conditions, which represents a robust criterion for unambiguous identification of iron in silicon. As the most sensitive technique to detect and determine an iron contamination in silicon heavily relies on the Fei and FeB defect parameters, the spectroscopic results are of special practical importance.

Charge-changing cross sections of fastH0,1+andHe0,1+,2+projectiles inC60

Electron capture and loss cross sections have been measured for fast light projectile ions of $0.5\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ ${\mathrm{H}}^{0,1+}$ and $0.5--2.0\phantom{\rule{0.3em}{0ex}}\mathrm{MeV}$ $^{4}\mathrm{He}^{0,1+,2+}$ in collisions with ${\mathrm{C}}_{60}$. The gaseous target of ${\mathrm{C}}_{60}$ was prepared by heating ${\mathrm{C}}_{60}$ powder in a target cell to temperatures of $300--500\phantom{\rule{0.2em}{0ex}}\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}$, and outgoing charge fractions were measured as a function of the cell temperature. Absolute cross sections are deduced by using two different vapor-pressure data available in literature. Experimental cross sections are examined in comparison with theoretical values obtained from various conventional formulas proposed for atomic targets. In addition, single- and double-electron capture cross sections are also calculated on the basis of a classical model by taking account of the local electron density of ${\mathrm{C}}_{60}$. From a complete set of our experimental cross sections, equilibrium charge fractions are also deduced and found to be essentially the same as carbon-foil data, indicating no gas-solid difference.

Single electron capture in slow collisions of H 2+ ions with H 2, O 2 and N 2 diatomic molecules

Collisions of the slow molecular hydrogen ions, H 2 + on H 2, O 2 and N 2 diatomic molecules in the energy range 700–2000 eV have been studied experimentally using the time-of-flight technique. The total cross sections for H 2 + with H 2, O 2 and N 2 are found to be in the range of (1.79 ± 0.27–2.78 ± 0.42) Å 2, (3.43 ± 0.51–5.15 ± 0.78) Å 2 and (3.46 ± 0.52–5.03 ± 0.75) Å 2, respectively. The measured electron capture cross sections show a slowly increasing behavior as a function of the incident energy. The measured cross sections for H 2 + with H 2 systems are compared with the existing data.

Osmium impurity-related deep levels in n-type GaAs

The 5d transition-metal impurity, osmium, has been incorporated during the growth of n-type GaAs epitaxial layers using low-pressure metal-organic chemical-vapor deposition to characterize defect states associated with this heavy and, therefore, thermally stable dopant impurity. Deep-level transient spectroscopy has been employed to investigate the electrical characteristics of Os-related deep levels. Two prominent deep levels have been identified with Os at Ec−0.28eV (Os1) and Ec−0.41eV (Os2) in the upper half-band-gap of GaAs, while no Os-related level has been clearly detected in the lower half-band-gap. The detailed characteristics determined for the two levels include thermal emission rate signatures, leading to the above cited thermal activation energies, electron-capture cross sections, and their temperature dependence, measured by direct pulse-filling technique and deep-level concentrations. Further, both levels are found to exhibit a significant dependence of thermal emission rates on the junction electric field. While Os1 does not allow accurate quantitative investigations of this field dependence due to its relatively low concentration, detailed quantitative data on the field dependence of the level Os2 are reported. Analysis of these data in light of the available theoretical models allows us to conclude that Os2 is probably a substitutional donor-type defect in GaAs.

Relative state selective capture cross sections for low energy collisions between highly charged bare ions and neutral atoms

Relative state selective (n) single electron capture cross sections are calculated for low energy collisions involving bare oxygen and nitrogen ions and neutral H and O, using a three body classical trajectory Monte Carlo (CTMC) method. The population of states are studied as a function of relative collision speed and ionization potential of the active target electron, and compared with other theoretical calculations and experimental data.

Deep defects and their electron-capture cross sections in polymorphous silicon-germanium thin films

Hydrogenated silicon-germanium alloys (SiGe:H) are deposited by plasma-enhanced chemical vapor deposition in a plasma regime close to that of the formation of powder. It is thus possible to obtain nanostructured materials that we call polymorphous materials, pm-Si1−xGex:H. Studies of space-charge-limited currents and space-charge relaxation allow to get information on midgap states originating from Ge. It is observed that the electron-capture cross section of states at the Fermi level increases linearly with their concentration. This is supported by modulated photocurrent experiments. Finally, it is shown that the variations of both the Ge dangling bond concentration and their electron-capture cross section with Ge content account for the photoresponse in these materials.

Target density effects in collisions of fast ions with solid targets

The influence of the target density on the charge-changing—ionization and electron capture—cross sections when fast ions penetrate through solid targets is considered. It is shown that, with the target density increasing, electron-capture cross sections decrease and ionization cross sections increase, resulting in a higher mean charge of exit ions after a solid target compared to a gas target (often called gas-solid or density effects).Recent experimental data on equilibrium charge-state fractions F∞q of 6.0 MeV u−1 ions with atomic numbers Z = 6–26 passing through carbon foils [10, 11] are described in terms of the calculated charge-changing cross sections using a four-charge state model. It is found that the reasonable agreement can be obtained provided the target-density effects are properly accounted for. Moreover, the present treatment allows one, for the first time, to estimate the distributions of the exit ions over excited nℓ states from experimental equilibrium charge-state fractions using the charge-changing cross sections calculated with the density effects included; here n and ℓ denote the principal and orbital quantum numbers, respectively.Numerical calculations of the charge-changing cross sections and equilibrium charge-state fractions of exit ions are performed for 6.0 MeV u−1 C2+, Ne4+, Mg5+, Si5+, Ar8+ and Fe9+ ions penetrating through carbon foils. Taking the target-density effects into consideration and comparing the calculated equilibrium charge fractions with the recently observed data for carbon foils, the nℓ distributions of exit ions with the largest fractions are estimated by fitting the calculated F∞q values to the experimental ones.

Recombination activity of iron-gallium and iron-indium pairs in silicon

The recombination activity of iron-gallium (FeGa) and iron-indium (FeIn) pairs in crystalline silicon is studied by means of injection-dependent carrier lifetime measurements on Fe-implanted, Ga- and In-doped p-type silicon wafers of different resistivities (0.3–15Ωcm). Compared to FeB pairs, FeGa and FeIn pairs are found to be much more effective recombination centers in p-type silicon. Using Shockley–Read–Hall statistics we determine the energy level Et of the FeGa-related center to be 0.20eV above the valence-band edge Eν. The strong recombination activity of FeGa is assigned to its large electron-capture cross section σn of 4×10−14cm2. The hole-capture cross section σp is 2×10−14cm2. For the FeIn-related recombination center, our measurements show that Et=Eν+0.15eV, σn=3.5×10−13cm2, and σp=1.5×10−14cm2. Strong illumination with white light is found to dissociate both types of pairs. Storage of the samples in the dark leads to a full repairing of FeGa and FeIn pairs. Lifetime changes measured before and after illumination can be used to determine the interstitial iron concentration in Ga- and In-doped silicon using calibration factors determined from the measured defect parameters.

Electronic properties of iron-boron pairs in crystalline silicon by temperature- and injection-level-dependent lifetime measurements

Iron-boron pairs in crystalline silicon are studied by measuring the recombination lifetime as a function of injection density, doping concentration, and temperature. The characteristic crossover point of the injection-level-dependent carrier lifetime curves measured before and after optical dissociation of the iron-boron pairs is analyzed to determine the energy level as well as the electron- and hole-capture cross sections of the acceptor level of iron-boron pairs, assuming known recombination parameters for interstitial iron. The doping concentration dependence of the crossover point gives an electron-capture cross section of (1.4±0.2)×10−14cm2, while the temperature dependence results in a hole-capture cross section in the range from 0.5×10−15to2.5×10−15cm2 and an energy level of (0.26±0.02)eV below the conduction-band edge.

Electron capture byNe4+ions from atomic hydrogen

Using the Oak Ridge National Laboratory ion-atom merged-beams apparatus, the absolute total electron-capture cross section has been measured for collisions of ${\mathrm{Ne}}^{4+}$ with hydrogen and deuterium at relative energies in the center-of-mass frame between 0.10 and 1006 eV/u. Comparison with previous measurements shows large discrepancies between 80 and 600 eV/u. For energies below $\ensuremath{\sim}1\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}$, a sharply increasing cross section is attributed to the ion-induced dipole attraction between the reactants. Multichannel Landau-Zener calculations are performed between 0.01 and 5000 eV/u and compare well to the measured total cross sections. Below $\ensuremath{\sim}5\phantom{\rule{0.3em}{0ex}}\mathrm{eV}∕\mathrm{u}$, the present total cross section calculations show a significant target isotope effect. At 0.01 eV/u, the H:D total cross section ratio is predicted to be $\ensuremath{\sim}1.4$ where capture is dominated by transitions into the ${\mathrm{Ne}}^{3+}$ $(2{s}^{2}2{p}^{2}3d)$ configuration.