Electron Capture Cross Sections Research Articles

Charge exchange in collisions of 1–100-keV Sn3+ ions with H2 and D2

Absolute cross sections for single electron capture by ${\mathrm{Sn}}^{3+}$ colliding with ${\mathrm{H}}_{2}$ and ${\mathrm{D}}_{2}$ have been measured and calculated in the energy range of 1--100 keV. The cross sections are determined by measuring the change in ion beam current with varying target density and by measuring the yields of charged target fragments by means of a time-of-flight spectrometer. The results for ${\mathrm{D}}_{2}$ show good agreement with our seven-state semiclassical calculations, while for ${\mathrm{H}}_{2}$ the experimental results increase more strongly than the calculations toward lower energies. This discrepancy is attributed to vibrational effects, not included in the calculations, that lead to the breakdown of the Franck-Condon approximation.

Total, state-selective, and angular-differential cross sections for electron capture in He2++H collisions in warm dense plasmas

The total, state-selective, and angular-differential cross sections for He2++H collision system in warm dense plasmas are studied by using the two-center atomic orbital close-coupling method in the energy range 0.1–300 keV/u. The calculations are performed for plasma density and temperature ranges ne ∼1018 to ∼1021 cm−3, Te = 0.3 eV–1.2 eV, typical for the H- and He-rich white dwarfs. The plasma environments are described by a unified screened potential involving electron degeneracy, finite-temperature gradient, and exchange-correlation effects. The results for H++H cases with the same plasma parameters are also presented for comparison to elucidate the discrepancies of plasma screening effects on the electron capture dynamics for collision systems with different nuclear symmetries. Moreover, classical Debye screening results are also given for comparison to clarify the effects of quantum correlations in warm dense plasmas on the electron capture dynamics. The present work is expected to provide theoretical and data support for the astrophysical plasmas.

MAVEN Observations of H− Ions in the Martian Atmosphere

AbstractAt Mars, charge exchange between solar wind protons and neutral exospheric hydrogen produces energetic neutral atoms (ENAs) that can penetrate into the collisional atmosphere, where they can be converted through collisions into H+ and H−. The Mars Atmosphere and Volatile EvolutioN mission observed a population of negatively charged particles at low altitudes, whose energies, angular distribution, and dependence on the upstream solar wind were consistent with H− produced by solar wind hydrogen ENAs. The highest fluxes of H− were observed near perihelion and the southern summer solstice. We calculated an average ratio of ∼4% between H− density and H+ density, implying a slightly smaller relative abundance than reported previously (∼10%). We found that the fraction of H ENAs converted to H− increases with the solar wind energy, in agreement with laboratory measurements of the H–CO2 electron capture cross section.

State-selective electron capture in collisions of fully stripped neon ions with ground-state hydrogen

Electron capture and ionisation in bare neon ion collisions with ground-state atomic hydrogen are modelled over the energy range from 1 to 2000 keV/u using the two-center semiclassical wave-packet convergent close-coupling method. The calculated total electron-capture cross section agrees very well with the molecular and atomic orbital close-coupling calculations at low and intermediate energies. Our results slightly overestimate the experimental results by Meyer et al [1985 Phys. Rev. A 32 3310], but underestimate the measurements by Panov et al [1983 Phys. Scr. T3 124] available only below 10 keV/u. At higher energies, where there are no measurements, the results also agree very well with the classical trajectory Monte-Carlo results. Partial n and nl-resolved electron-capture cross sections, important for fusion plasma diagnostics, have also been calculated for final states up to n = 10, where n and l are the final state principal and angular momentum quantum numbers, respectively. The results are generally in good agreement with the atomic calculations. However, due to the finer energy grid used, we are able to detect pronounced oscillations in the state-selective cross sections for n ⩾ 8 at energies below 10 keV/u. Our results for the total ionisation cross section are overall in good agreement with the latest classical trajectory Monte-Carlo results.

Charge transfer of potassium by the impact of proton particle

The total cross section for electron capture is being calculated in this paper for 2s state of hydrogen atom in proton-potassium collision. The energy range chosen for the calculation is 50 keV to 10,000 keV and we have used Coulomb projected Born (CPB) approximation method. Also the differential cross section at different scattering angle has been calculated. The result obtained are compared with available theoretical data and found it to be in good agreement.

State-selective electron capture cross sections in collision between fully striped ions and ground state hydrogen atom—Classical treatment of the collision

We present a state-selective electron capture cross sections database from the ground state hydrogen atom. A standard three-body classical trajectory Monte Carlo (CTMC) and quasi-classical trajectory Monte Carlo (QCTMC) models were employed for impact energies between 10 and 200 keV/amu. The projectile ions are H+, He2+, Li3+, Be4+, B5+, C6+, N7+, and O8+. The cross sections are tabulated for each value of the final quantum numbers n, l, m, summed over m for the given n and l and over l and m for the given n. The maximum quantum number nmax is set to 4 for H+, He2+ and Li3+, as well as to 5 for Be4+, B5+, C6+, N7+ and O8+, respectively.

On the nature of thermally activated defects in n-type FZ silicon grown in nitrogen atmosphere

n-type float-zone silicon grown in a nitrogen atmosphere contains defects which are activated by temperatures between 450 and 700 °C. We use deep level transient spectroscopy (DLTS) to study the nature of these defects and the impact of the nitrogen content and the polysilicon feed stock type. We find four dominant DLTS peaks with activation energies of Ena = 0.16 eV (E1), Ena = 0.21 eV (E2), Ena = 0.34 eV (E4), and Ena = 0.64 eV (E6). We tentatively assign the two DLTS peaks E1 and E2 to single acceptor and single donor levels of the same defect, a complex of nitrogen with an impurity. Furthermore, we tentatively assign the two DLTS peaks labeled E4 and E6 to two levels of the off-center substitutional nitrogen. Based on the apparent electron capture cross sections and an analysis of the electric field effect on the emission rates, we propose them to be double and single acceptor levels, respectively. Due to its position at midgap and the competing electron and hole emission, the apparent concentration of E6 is reduced to one fifth of the total defect concentration. Correcting for these processes, we find the activation energies for electron and hole emission to be En = 0.50 eV and Ep = 0.68 eV.

Effective one-electron approach to proton collisions with molecular hydrogen

The two-centre wave-packet convergent close-coupling approach to ion–atom collisions is extended to study proton collisions with molecular hydrogen including electron-capture channels. We use a model potential to represent the molecular target as an effective one-electron spherically symmetric system. This greatly simplifies the target structure, allowing us to use already existing code developed for ion collisions with single-electron targets. Calculated total cross sections for electron capture, single ionisation, and excitation processes generally agree well with experimental data and other theoretical calculations where available. However, the total electron capture cross section is found to overestimate the experimental data at low energies, while the total ionisation cross section is slightly underestimated. Additionally, we present state-resolved cross sections for capture into the 1s, 2ell , and 3ell states of the projectile where deviation between various previous calculations is substantial. Our results lead to overall improvement over previous theoretical studies although discrepancies with experiment are observed for 3p and 3d capture. We conclude that treating molecular hydrogen as an effective one-electron system within the two-centre coupled-channel approach to one-electron targets can give reasonably accurate total cross sections at intermediate and high energies, without the need for a complex and computationally demanding two-electron target representation.

Classical and semiclassical calculations of state-selective cross sections for electron capture and excitation in Be4++H(2s) collisions

A computational study of ${\mathrm{Be}}^{4+}+\mathrm{H}(2s)$ collisions has been carried out. Two computational models have been employed: the classical trajectory Monte Carlo (CTMC) method and the numerical solution of the time-dependent Schr\"odinger equation (GTDSE). The integral $n$ and $nl$ partial cross sections for H excitation and electron capture, obtained with both methods, are compared at two energies: 20 and 100 keV/u. It is shown that the CTMC, with an improved hydrogenic initial distribution, provides excitation cross sections in good agreement with the numerical calculation for excitation to $\mathrm{H}(n$) with $n>3$. The agreement between the corresponding $nl$ partial cross sections from both methods is less satisfactory at 100 keV/u, where there is a transition from the low-energy mechanism that involves an increase of the populations with $l$, and the high-energy mechanism, where the dipole-allowed transitions are dominant. The electron capture cross sections calculated with the CTMC method do not depend on the initial distribution and show a reasonable agreement with the GTDSE ones, which supports the use of the CTMC method to calculate electron capture cross sections into highly excited levels and total cross sections. The mechanism of the electron capture process is discussed and CTMC calculations of the ionization process are also presented.

Charge transfer in collisions of H<sup>+</sup>, Li<sup>3+</sup>, Be<sup>4+</sup> and O<sup>7+</sup> ions with He atom based on 4-classical trajectory Monte Carlo method

The classical trajectory Monte Carlo (CTMC) method is a common method to study the charge-transfer and impact-ionization cross sections for the collisions between ions and atoms, and the heavy particle collision in astrophysics and laboratory plasma environment. Here in this work, we use the 4-CTMC method to study a four-body collision process including two bound electrons, and the Hamiltonian equation of the four-body dynamic system is solved numerically. The single/double electron ionization and capture cross sections are calculated for collisions of high charge state ions (Li<sup>3+</sup>, Be<sup>4+</sup> and O<sup>7+</sup>) with helium atom in a wide range of projectile energy. The calculation results show that the results from the 4-CTMC method and the experimental measurements are in better agreement in a projectile energy range of 50－200 keV/amu for proton-helium collision system. In addition, for incident ions with high charge state, the results calculated by the 4-CTMC method are in better agreement with the experimental measurements or other theoretical values in a projectile energy range of 100－500 keV/amu. Though the double ionization and capture cross sections calculated by 4-CTMC or 3-CTMC method are higher than the experimental results due to ignoring the electron correlation, the results from the 4-CTMC method are in better agreement with the experimental results.

Analytical model of the modulated photoluminescence in semiconductor materials

Modulated photoluminescence (MPL) is an optoelectronic characterization technique of semiconductor materials. Going to high frequencies enables one to characterize fast phenomena, and so materials with a short lifetime such as chalcogenides or III–V absorbers. Some typical signatures have already been experimentally observed. However, physical mechanisms and quantitative analyses are not well understood yet. Here, using both an analytical approach and a full numerical modeling, we study how the energy position of a defect level, its electron and hole capture cross sections, its density, influence the frequency dependence of the MPL phase. We show that quantitative information can be extracted. We also study the effect of additional surface recombination, and of non homogeneities created by carrier generation profiles or asymmetric top surface and bottom surface recombination velocities, where diffusion of the carriers plays a role and can be limiting at high frequency. Finally we apply our model to an experimental result to extract defect parameters of the sample. Our analysis highlights the usefulness of MPL and the importance of having a proper modeling of the experiment.

State selective classical electron capture cross sections in Be4+ \u2009+\u2009H(1s) collisions with mimicking quantum effect

We present state-selective electron capture cross sections in collision between Be4+ and ground state hydrogen atom. The n- and nl-selective electron capture cross sections are calculated by a three-body classical trajectory Monte Carlo method (CTMC) and by a classical simulation schema mimicking quantum features of the collision system. The quantum behavior is taken into account with the correction term in the Hamiltonian as was proposed by Kirschbaum and Wilets (Phys Rev A 21:834, 1980). Calculations are carried out in the projectile energy range of 1–1000 keV/amu. We found that our model for Be4+ + H(1s) system remarkably improves the obtained state-selective electron capture cross sections, especially at lower projectile energies. Our results are very close and are in good agreement with the previously obtained quantum–mechanical results. Moreover, our model with simplicity can time efficiently carry out simulations where maybe the quantum mechanical ones become complicated, therefore, our model should be an alternative way to calculate accurate cross sections and maybe can replace the quantum–mechanical methods.

Evaluation of dominant loss mechanisms of PERC cells for optimization of rear passivating stacks

In this paper a comprehensive theoretical analysis has been carried out for Passivated Emitter Rear Contact (PERC) solar cells with the help of numerical modeling using Sentaurus TCAD simulation software. For the rear passivation we have explored various dielectric layers, such as, Aluminium Oxide (Al2O3), Silicon Oxide (SiO2), Hafnium Oxide (HfO2) and Aluminium Nitride (AlN) along with the Silicon Nitride (SiNx) capping layer. Optical generation near the rear side the passivating stacks was analyzed. Recombination current densities were also evaluated for assorted defects with various concentration, hole and electron capture cross sections and position of quasi Fermi level for surface passivation. Thicknesses of the passivating and capping layers were optimized with respect to optical and electrical losses. Field effect passivation (FEP) property of the rear passivating stacks was also investigated. For simulated textured Al-BSF solar cell on silicon wafer of bulk lifetime 250µs, an efficiency of 19.5% was obtained. This solar cell has been used here for reference. We have achieved efficiency of 22.6% for PERC solar cells of rear contact area of 8% on similar type of wafer. It was noticed from few articles that industrial PERC solar cells were fabricated using rear metal contact less than 10% which is well validating our study. It was also perceived that employing Passivated Emitter Rear Totally Diffused (PERT) structure with optimized design parameters further improved the efficiency to ∼ 24%.

Integrated total and state-selective cross sections for bare beryllium ion collisions with atomic hydrogen

We apply the two-centre wave-packet convergent close-coupling approach to calculate integrated total and state-selective cross sections for electron capture and ionisation in bare beryllium ion collisions with atomic hydrogen. This is done in the energy region between 1 keV/u to 1 MeV/u. Good agreement was achieved in comparison to previous theoretical works for the total electron capture cross section across all energies, however the total ionisation cross section shows disagreement with preceding calculations. We present accurate n and ℓ-resolved (n and ℓ are the principal and the angular momentum quantum numbers of the final state in electron capture processes, respectively) cross sections required for plasma impurity diagnostics where there have been discrepancies between different approaches for n > 5. Otherwise, excellent agreement between our results and previous calculations of state-selective cross sections for electron-capture is observed for a wide number of different n. The calculations have been performed on a fine energy mesh to identify oscillatory structures in the n- and nl-resolved capture cross sections in the low-energy region. Our results show pronounced oscillations in the cross sections for n ⩾ 5 below 20 keV/u.

Electron-capture cross sections in collisions of $${\mathrm{He}}^{+}$$ with several molecules

Electron-capture cross sections in collisions of $${\mathrm{He}}^{+}$$ with several molecules

Empirical relations for heavy-ion equilibrated charges and charge-changing cross sections in diluted $$\hbox {H}_{{2}}$$ with application

Highly ionized heavy evaporation residues (ERs) resulting from heavy-ion (HI) fusion–evaporation reactions are knocked out from solid targets to rarefied gas of gas-filled recoil separators. In gas, these ERs undergo charge-changing collisions on their way to a detection system. An equilibrium between the loss of charge (electron capture) and the gain of charge (electron loss) for ionized ERs allows one to use equilibrated charge-state distributions in trajectory calculations for ERs moving through a magnetic field. The distance from a target to the point where the ER charge-state distributions become to be the equilibrated one is essential for such calculations. This distance can be estimated in simulations based on electron capture and loss cross sections for energetic HIs. Reasonable approximations of available experimental data have provided these cross sections, which were applied to the Monte Carlo simulations of the ionic charge evolution for highly ionized heavy ERs. The results of these simulations demonstrate the setting of charge equilibration close to the target.

Field-Effect Passivation of Undiffused Black Silicon Surfaces

Black silicon (b-Si) surfaces typically have a high density of extreme nanofeatures and a significantly large surface area. This makes high-quality surface passivation even more critical for devices such as solar cells with b-Si surfaces. It has been hypothesized that conformal dielectrics with a high fixed charge density ( <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>f</b></sub> ) are preferred as the nanoscale features of b-Si result in a significant enhancement of field-effect passivation. This article uses 1-D, 2-D, and 3-D numerical simulations to study surface passivation of b-Si, where we particularly focus on the charge carrier control by | <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>f</b></sub> | up to 1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">13</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> under accumulation conditions. We will show that there is a significant space charge region compression in b-Si nanofeatures, which affects the charge carrier population control for moderate | <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>f</b></sub> | up to ≈1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> . The average surface minority charge carrier density can be reduced by 70% in some cases, resulting in an equivalent reduction in area-normalized surface recombination losses if the effective surface recombination velocity ( <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ) is limited by minority carriers. This provides a possible solution for the empirical <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> ∝ 1/ <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>f</b></sub> <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">4</sup> reported previously. We will also show that the situation is more complicated for surface passivation films where the ratio between the electron and hole capture cross section (σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>n</b></sub> / σ <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>p</b></sub> ) is higher than 10 for p-type surfaces. For commonly used surface passivation films with a | <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">Q</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><b>f</b></sub> | larger than ≈1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">12</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-2</sup> , there is little space charge compression for b-Si. Consequently, <b xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">S</b> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">eff</sub> simply scales with the surface area, i.e., there is no enhanced reduction of surface recombination by field-effect passivation on b-Si.

High-resolution Laboratory Measurements of K-shell X-Ray Line Polarization and Excitation Cross Sections in Helium-like S XV Ions

Abstract We report measurements of electron-impact excitation cross sections for the strong K-shell n = 2 → 1 transitions in S xv, using the LLNL EBIT-I electron beam ion trap, two crystal spectrometers, and the EBIT Calorimeter Spectrometer. The cross sections are determined by direct normalization to the well-known cross sections of radiative electron capture, measured simultaneously. Using contemporaneous polarization measurements with the two crystal spectrometers, whose dispersion planes are oriented parallel and perpendicular to the electron beam direction, the polarization of the direct excitation line emission is determined, and in turn the isotropic total cross sections are extracted. We further experimentally investigate various line-formation mechanisms, finding that radiative cascades and collisional inner-shell ionization dominate the degree of linear polarization and total line-emission cross sections of the forbidden line, z.

Contribution of the carbon-originated hole trap to slow decays of photoluminescence and photoconductivity in homoepitaxial n-type GaN layers

N-type GaN epitaxial layers grown via metal organic vapor-phase epitaxy typically exhibit a yellow luminescence (YL) band owing to carbon-related deep levels in the photoluminescence spectra. The decay of YL after pulse excitation involves a long time constant (∼0.2 ms at room temperature), whereas microwave photoconductivity decay (μ-PCD) curves show the corresponding component of the time constant. To clarify the origin of the long decay time, the temperature-dependent time constants of YL decay and μ-PCD curves are analyzed using a numerical model based on rate equations for trapping and emission through a deep level. The characteristics of the decays are well reproduced by a recombination model using a hole trap H1 at an energy of EV + 0.88 eV because of the acceptor-like state of carbon on a nitrogen site (CN) whose electron capture cross section (σn) is estimated to be 3 × 10−21 cm2. The slow decay in μ-PCD signals indicates that the electrons before being captured to H1 traps are free electrons in the conduction band. These findings indicate that the slow recombination process through CN results in tail currents in the turn-off switching periods of devices.

Single-Photon Sources Based on Novel Color Centers in Silicon Carbide P–I–N Diodes: Combining Theory and Experiment

HighlightsTheory of electrically driven single-photon sources based on color centers in silicon carbide p–i–n diodes.New method of determining the electron and hole capture cross sections by an optically active point defect (color center) from the experimental measurements of the single-photon electroluminescence rate and second-order coherence.The developed method is based on the measurements at the single defect level. Therefore, in contrast to other approaches, one point defect is sufficient to measure its electron and hole capture cross sections.Point defects in the crystal lattice of SiC, known as color centers, have recently emerged as one of the most promising single-photon emitters for non-classical light sources. However, the search for the best color center that satisfies all the requirements of practical applications has only just begun. Many color centers in SiC have been recently discovered but not yet identified. Therefore, it is extremely challenging to understand their optoelectronic properties and evaluate their potential for use in practical single-photon sources. Here, we present a theoretical approach that explains the experiments on single-photon electroluminescence (SPEL) of novel color centers in SiC p–i–n diodes and gives the possibility to engineer highly efficient single-photon emitting diodes based on them. Moreover, we develop a novel method of determining the electron and hole capture cross sections by the color center from experimental measurements of the SPEL rate and second-order coherence. Unlike other methods, the developed approach uses the experimental results at the single defect level that can be easily obtained as soon as a single-color center is identified in the i-type region of the SiC p–i–n diode.Graphic