Charge Transfer Cross Sections Research Articles

Theoretical study of resonances formed in low-energy Li− + H collisions

The full quantum molecular orbital close-coupling (QMOCC) method and optical potential method have been employed to study the nonradiative charge transfer and radiative decay processes for Li− colliding with H atoms. The cross sections of these processes are calculated rigorously in the energy range of 0.1–1.2 eV. It is found that rich and significant resonance structures appear at the same positions in the cross sections for both nonradiative charge transfer and radiative decay processes. The primary resonances are identified and studied by QMOCC (by fitting the eigenphase sum of the K-matrix) and complex absorbing potential (CAP) method. The resonance parameters calculated by CAP agree very well with those extracted from QMOCC calculations.

Quasi-four-body treatment of charge transfer in the collision of protons with atomic helium: II. Second-order non-Thomas mechanisms and the cross sections

A fully quantum mechanical four-body treatment of charge transfer collisions between energetic protons and atomic helium is developed here. The Pauli exclusion principle is applied to both the wave function of the initial and final states as well as the operators involved in the interaction. Prior to the collision, the helium atom is assumed as a two-body system composed of the nucleus, He2+, and an electron cloud composed of two electrons. Nonetheless, four particles are assumed in the final state. As the double interactions contribute extensively in single charge transfer collisions, the Faddeev-Lovelace-Watson scattering formalism describes it best physically. The treatment of the charge transfer cross section, under this quasi-four-body treatment within the FWL formalism, showed that other mechanisms leading to an effect similar to the Thomas one occur at the same scattering angle. Here, we study the two-body interactions which are not classically described but which lead to an effect similar to the Thomas mechanism and finally we calculate the total singlet and triplet amplitudes as well as the angular distributions of the charge transfer cross sections. As the incoming projectiles are assumed to be plane waves, the present results are calculated for high energies; specifically a projectile energy of 7.42 MeV was assumed as this is where experimental results are available in the literature for comparison. Finally, when possible we compare the present results with the other available theoretical data.

Charge transfer in low-energy collisions of H with He+ and H+ with He in excited states

The charge transfer process in collisions of excited (n = 2, 3) hydrogen atoms with He+ and in collisions of excited helium atoms with H+ is studied theoretically. A combination of a fully quantum-mechanical method and a semi-classical approach is employed to calculate the charge-exchange cross sections at collision energies from 0.1 eV u−1 up to 1 keV u−1. These methods are based on accurate ab initio potential energy curves and non-adiabatic couplings for the molecular ion HeH+. Charge transfer can occur either in singlet or in triplet states, and the differences between the singlet and triplet spin manifolds are discussed. The dependence of the cross section on the quantum numbers n and l of the initial state is demonstrated. The isotope effect on the charge transfer cross sections, arising at low collision energy when H is substituted by D or T, is investigated. Rate coefficients are calculated for all isotopes up to 106 K. Finally, the impact of the present calculations on models of laboratory plasmas is discussed.

Line ratios for x-ray emission in O8+ collisions with H and He atoms

A time-dependent lattice method is used to calculate state selective charge transfer cross sections in O8+ collisions with H and He atoms at incident energies of 1.00, 4.00, and 8.18 keV amu−1. Using standard radiative transitions, Lyman (β/α), Lyman (γ/α), and Lyman (δ/α) line ratios are calculated using time-dependent lattice capture cross sections for both H and He. The line ratios for He are found to be in good agreement with direct experimental measurements at 3.11 keV amu−1.

Orientation Dependence of The Charge Transfer Cross Section in the Collision of Positrons With H2 + Under the First Born Approximation and Implementing a Fourier Transform technique

Charge transfer from the hydrogen molecular ion by positron impact is studied using Fourier transform techniques. The interaction amplitude and the differential cross section for the reaction are obtained within the first born approximation. The interference patterns and the effect of the orientation angle of the molecular axis are analyzed, and compared with the available data in the literature.

Differential and total cross sections for charge transfer and transfer-excitation in ion-helium collisions

Total cross sections for single charge transfer in collisions of multicharged bare ions with ground-state helium atoms at incident energy ranging from 40 to 5000 keV/amu have been calculated in the framework of a four-body model of final channel distorted-wave (FC-DW-4B) approximation. In this formalism, distortion in the final channel related to the Coulomb continuum of the target and the Coulomb interaction between the passive electron in the target with the projectile are included. In all cases, total single electron-capture cross sections have been calculated by summing over all contributions up to $n=3$ shells and subshells. It has been observed that the contribution of the capture cross sections into excited states have insignificant contributions for symmetric collisions. Comprehensive comparisons are made between the four body model of boundary corrected continuum intermediate-state approximations [Phys. Rev. A 83, 032706 (2011)] and the present FC-DW-4B model. The main purpose of the present study is to investigate the relative importance of dynamic electron correlation and the role of passive electron in the target at intermediate and high impact energies. In addition, projectile angular differential cross sections (DCS) for charge transfer and transfer-excitation in $p\text{-He}$ collisions are calculated at different impact energies. At low projectile energies, the present DCS data exhibits the typical steeply decreasing dependence on the projectile scattering angles, whereas at high impact energies, the double-scattering region centered on the Thomas angle is obtained. Detailed comparisons with the available experimental data and other theories are reported with the purpose of further assessing the relevance of the present model at different impact energies. Overall, the calculated cross sections show good agreement with the available experimental findings.

Ab initio study of charge transfer in low energy collisions of B4+ with H

The charge transfer processes in collisions of B4+(1s) ions with H(1s) atoms are investigated by using the quantum-mechanical molecular orbital close-coupling method with electron translational effects included in the impact energy region of 10-5 − 20 keV/u. Molecular data with high accuracy are calculated using ab initio method. Our calculations clarified the controversy in the total and state-selective cross sections at low energies. The treatment of the core electron will influence the accuracy of the calculated molecular structure and then sensitively influence the charge transfer cross sections in the low energy region. The rotational couplings play an important role in the state-selective cross sections at energies above 50 eV/u, but weakly influence the total cross sections for this collision system.

Reduced mobility of He+ in CF4

This paper is devoted to a presentation of a cross section set for the scattering of He+ ions in CF4, which is assessed by using available experimental data for exothermic charge transfer cross sections that produce and ions and endothermic charge transfer cross sections that produce CF+, C+ and F+ ions. Due to the significant particle losses, experimental transport coefficients have not been measured. Transport properties of He+ ions in CF4 needed for modeling discharges containing mentioned ions are calculated by the Monte Carlo method at a temperature of T = 300 K. Significant differences between flux and bulk transport coefficients are noticed, which is important for fluid models that exploit flux transport coefficients as input data.

Radiative charge transfer in collisions of C with He+

Radiative charge exchange collisions between a carbon atom and a helium ion , both in their ground state, are investigated theoretically. Detailed quantum chemistry calculations are carried out to obtain potential energy curves and transition dipole matrix elements for doublet and quartet molecular states of the HeC+ cation. Radiative charge transfer cross sections and rate coefficients are calculated and are found at thermal and lower energies to be large compared to those for direct charge transfer. The present results might be applicable to modelling the complex interplay of (or ), , and at the boundaries of interstellar photon dominated regions and in x-ray dominated regions, where the abundance of affects the abundance of .

Confinement effects on the electron transfer cross section: a study of He2+ colliding on atomic H

In this work, the role of pressure produced by the surrounding conditions of neighboring atoms on the charge transfer cross section in the interaction of ions with atoms is studied. The effect of pressure on the target is simulated by a spherical confinement cavity model with the target atom at its center. The electron transfer probability is obtained by a time-dependent solution to the Schrödinger equation by means of a finite-difference approach and the Crank–Nicolson propagation method. Results are presented for the benchmark system HeH() under different confinement conditions. Within this first-order approach to pressure effects, it is found that the radial region of the electron capture probability is affected by the cavity, meanwhile the rotational part of the interaction is only slightly modified. Furthermore, Stückelberg oscillations are preserved as a function of the impact parameter and only its intensity is reduced with a slight shift towards low impact parameters as pressure is increased. The pressure effect is larger when the electron interaction on the target is restricted to within the cavity than when it acts also outside the cavity. In addition, the larger the pressure, the smaller the electron capture cross section. The numerical results are interpreted by an analytic two-state model that supports the numerical findings.

Influence of microhydration on the structures and proton-induced charge transfer in RNA intermediates.

Solvation effects are of major interest in the context of radiation damage, due to their potential applications in cancer therapy. Reliable modeling of the solvent is, however, quite challenging, and numerous studies have been devoted to isolated biomolecules and stepwise-hydrated molecules in which the amount of solvent is controlled one molecule at a time. The influence of stepwise hydration on radiation damage is investigated here using the example of proton-induced charge transfer in two biomolecular targets. Uracil has been widely investigated both experimentally and theoretically in this context, and 2-aminooxazole was recently shown to be a potentially important intermediate in prebiotic chemistry. Focusing here on doubly hydrated biomolecules, stable structures and infrared spectra were obtained by combining the results of molecular dynamics simulations with those of quantum chemistry calculations performed at the density-functional theory level with the double hybrid M06-2X functional. The charge-transfer cross-sections upon proton impact were obtained from ab initio molecular calculations and after applying a semi-classical approach to investigate the collision. Our results suggest a significant relationship between the detailed hydration structure and the efficacy of proton-induced charge transfer, highlighting the competing roles of inter- and intramolecular hydrogen bonding.

Charge transfer in collisions of H+ with Li (1s22s, 2pz): TD-MADNESS approach

In this work we carry out a study of the single-electron charge transfer process for H+ collisions with atomic neutral lithium, in its ground and first excited state, at 1−25 keV amu−1. For this, we solve numerically the time dependent Schrödinger equation to the defined accuracy by using a multi-resolution adaptive approach, thus removing the uncertainties connected to a basis size and spatial and temporal numeric mesh size. We approximate the atomic lithium target by a single electron model in a frozen-core pseudo-potential while the projectile follows a straight line trajectory. Within these approximations we report new benchmark data for charge transfer cross sections to n = 2, and 3 states of hydrogen from 1s22s and 1s22pz of Li. Available theoretical and experimental data in the literature are in reasonable agreement with our results.

Laser-modified charge-transfer processes in He2++H collisions

Laser-modified electron capture processes are studied for the He2++H collision system in the low projectile energy region by solving the time-dependent Schrödinger equation. An obvious enhancement of the total and dominant state-selective charge-transfer cross sections are observed with respect to laser-free and weaker laser cases for low projectile energy. The influence of the laser pulse phase upon the cross sections is also studied and the reduction of the cross section is observed at some pulse phases with the increasing of the projectile energy. The time evolution for the dynamics of the collision system is analyzed in order to explore the dependence of the cross sections upon the pulse phase.

Stepwise Hydration of 2-Aminooxazole: Theoretical Insight into the Structure, Finite Temperature Behavior and Proton-Induced Charge Transfer.

It was recently suggested that 2-aminooxazole (AO) could contribute to the formation of RNA nucleotides on primitive earth. In this article we have considered by means of computational modeling the influence of microhydration on the structural and spectral properties of this potential prebiotic molecule. The stable structures of AO(H2O)n were obtained first by sampling the potential energy landscapes of clusters containing up to n = 20 water molecules, using a simple but reasonably accurate force field and replica-exchange molecular dynamics simulations. Through reoptimization using an explicit description of electronic structure at the level of density functional theory with the M06-2X functional, the formation energies, ionization energies and electron affinities were determined in the vertical and adiabatic treatments, as well as vibrational and optical spectra covering the far-IR, mid-IR, and lower part of the UV ranges. The results generally show a clear segregation between the aminooxazole solute and the water molecules, a water cluster being formed near the nitrogen and amino group side leaving the hydrocarbon side dry even at temperatures corresponding to the liquid state. The spectral signatures generally concur and show distinct contributions of the solute and solvent, spectral shifts to lower energies being in agreement with earlier calculations in bulk solvent. We have also investigated the importance of microhydration on the charge transfer cross section upon collision with a proton, thereby extending an earlier investigation on the bare AO molecule. The presence of water molecules generally reduces the propensity for charge transfer at small sizes, but the influence of the solvent steadily decreases in larger droplets.

Elastic, charge transfer, and related transport cross sections for proton impact of atomic hydrogen for astrophysical and laboratory plasma modeling

Updating and extending previous work (Krstić and Schultz 1999 J. Phys. B: At. Mol. Opt. Phys. 32 3458 and other references) comprehensive calculations were performed for elastic scattering and charge transfer in proton—atomic hydrogen collisions. The results, obtained for 1301 collision energies in the center-of-mass energy range of 10−4–104 eV, are provided for integral and differential cross sections relevant to transport modeling in astrophysical and other plasma environments, and are made available through a website. Use of the data is demonstrated through a Monte Carlo transport simulation of solar wind proton propagation through atomic hydrogen gas representing a simple model of the solar wind interaction with heliospheric neutrals.

Classical analysis of angular differential and total cross sections for charge transfer and ionization in He-like systems-helium atom collisions

A classical approach is used to determine the angular differential and total cross sections for charge transfer and ionization in He-like systems (Li+, Be2+, B3+, C4+, N5+, O6+) and helium atom collisions. It is found that the classical approach, i.e., classical trajectory Monte Carlo method (CTMC), gives adequate results for charge transfer and ionization in He-like systems and helium atom collisions. A model potential has been used for interactions of the active electrons with the projectile and target ion. We also observed sudden increase in the ionization cross sections at lower velocities, and it is explained in terms of quasi-molecular formation and multiple interactions. The behavior of angular differential cross sections calculated at very small scattering angle (<0.1°) in all systems shows a forward peak followed by an asymptotic fall with oscillatory structure at higher angles. These oscillations were classically interpreted in terms of swapping. The calculated cross sections are found in reasonable agreement with the available experimental and theoretical results.

Micrometer-sized negative-ion accelerator based on ultrashort laser pulse interaction with transparent solids

We report here energetic (>100 keV) negative hydrogen ions (H(-)) generated in the interaction of moderately intense (10(18) W cm(-2)) ultrashort laser pulses (45 fs) with transparent hydrogen containing solid targets. An unambiguous and consistent detection of negative hydrogen ions, with a flux of 8×10(11)H(-) ions/sr, has been observed in every single laser shot, using a Thomson parabola ion spectrograph. Simple estimates based on charge transfer cross sections match well with experimental observations. Our method offers the implementation of an intense, ultrashort laser based negative-ion source at a higher repetition rate, which can be important for various applications.

Effect of electron-nuclei interaction on internuclear motions in slow ion-atom collisions

The electron-nuclei interaction affects the internuclear motion in slow ion-atom collisions, which in turn affects theoretical results for the cross sections of various collision processes. The results are especially sensitive to the details of the internuclear dynamics in the presence of a strong isotope effect on the cross sections, as is the case, e.g., for the charge transfer in low-energy collisions of ${\text{He}}^{2+}$ with H, D, and T. By considering this system as an example, we show that internuclear trajectories defined by the Born-Oppenheimer (BO) potential in the entrance collision channel, which effectively accounts for the electron-nuclei interaction, are in much better agreement with trajectories obtained in the ab initio electron-nuclear dynamics approach [R. Cabrera-Trujillo et al., Phys. Rev. A 83, 012715 (2011)] than the corresponding Coulomb trajectories. We also show that the use of the BO trajectory instead of the Coulomb trajectory in the calculations of the charge-transfer cross sections within the adiabatic approach improves the agreement of the results with ab initio calculations.

Single charge transfer in collisions with H and He atoms

A time-dependent lattice (TDL) method is used to calculate state selective charge transfer cross sections in collisions with H and He atoms. The (nl) capture cross sections for energies of 2.7, 5.2, and 8.3 keV amu−1 are found to be in good agreement with recent atomic-orbital close-coupling (AOCC) calculations for H and somewhat larger than previous AOCC calculations for He. Using standard radiative transition rates, Lyman β/α and γ/α line ratios are calculated using TDL and AOCC capture cross sections for both H and He. The theoretical line ratios for He are found to bracket recent experimental measurements.

Resonant charge transfer in slow Li+−Li(2s) collisions**Project supported by the National Natural Science Foundation of China (Grant Nos. 11179041, 11474032, and 11474033) and the NSAF (Grant No. U1330117).

The resonant charge transfer process for Li+−Li(2s) collision is investigated by the quantum-mechanical molecular orbital close-coupling (QMOCC) method and the two-center atomic-orbital close-coupling (AOCC) method in an energy range of 1.0 eV/u–104 eV/u. Accurate molecular structure data and charge transfer cross sections are given. Both the all-electron model (AEM) and one-electron model (OEM) are used in the QMOCC calculations, and the discrepancies between the two models are analyzed. The OEM calculation can also give a reliable prediction of the cross sections for energies below 1 keV/u.