Charge Transfer Cross Sections Research Articles

Elastic and electron capture processes in slow He+–He collision

Aims. The elastic and electron-capture processes of He+ ions with ground helium atoms are very important for studies in astrophysics. It is essential to have reliable state-selective charge transfer, elastic, and transport cross-sections, along with the corresponding reaction rate coefficient data, especially for low collision energies. Methods. We investigated the elastic and non-radiative electron-capture processes in He+(1s)–He(1s2) collisions are investigated employing the full quantum-mechanical molecular orbital close-coupling method. The adopted ab initio adiabatic potentials and coupling matrix elements were obtained by a multi-reference single- and double-excitation configuration interaction approach. Results. We computed the elastic, charge-transfer, and transport cross-sections in the energy range of 0.01–2500eVamu−1 and the reaction rate coefficient in the temperature range of 10–109 K. Good agreement was achieved when compared to the available experimental and theoretical results. Shape resonance, Regge, and Glory oscillations were also found in the elastic and charge transfer cross-sections in the energy region considered here.

Charge transfer and ionization cross-sections in collisions of singly charged lithium ions with helium and nitrogen atoms

We present a non-perturbative classical treatment of the charge transfer and ionization processes in collisions between singly charged lithium ions with helium and nitrogen atomic targets. Single capture and single ionization total cross sections are calculated using a three-body classical trajectory Monte Carlo (CTMC) method in which the interaction between the collision partners is described by a Garvey-type model potential. The cross sections are evaluated for collision energies between 20 keV and 100 MeV. In particular, we found excellent agreement between our results and the available experimental data for the case of the single capture of He(1s) by Li+ ions. In addition, our CTMC results are in a reasonable agreement with the experimental results for collision energies higher than 200 keV for single capture of N(2p) atoms by Li+. Furthermore, we present single ionization cross sections for both collision systems.

Charge transfer cross sections and transport coefficients of 39K+ − 39K and 41K+ − 39K at low-energy

The transport coefficients, diffusion D(T), and reduced mobility K0 are calculated for systems 39K+ − 39K and 41K+ − 39K in a thermal range between 1 and 3000 K within the Chapman–Enskog approximation. This is preceded by a theoretical study of the collision of alkaline ions in their gases for low and intermediate energies in the range 10−14 ≤ E ≤ 10−3 (a.u.), and within this field, we calculate the interaction energy potentials and elastic and charge-transfer cross sections and discuss the isotopic effects in the 39 K+ − 39K and 41K+ − 39K collisions.

Charge Transfer and Electron Production in Proton Collisions with Uracil: A Classical and Semiclassical Study.

Cross sections for charge transfer and ionization in proton-uracil collisions are studied, for collision energies 0.05<E<2500 keV, using two computational models. At low energies, below 20 keV, the charge transfer total cross section is calculated employing a semiclassical close-coupling expansion in terms of the electronic functions of the supermolecule (H-uracil)+. At energies above 20 keV, a classical-trajectory Monte Carlo method is employed. The cross sections for charge transfer at low energies have not been previously reported and have high values of the order of 40 Å2, and, at the highest energies of the present calculation, they show good agreement with the previous results. The classical-trajectory Monte Carlo calculation provides a charge transfer and electron production cross section in reasonable agreement with the available experiments. The individual molecular orbital contributions to the total electron production and charge transfer cross sections are analyzed in terms of their energies; this permits the extension of the results to other molecular targets, provided the values of the corresponding orbital energies are known.

Characteristic energy and reduced mobility for Ne+ ions in CF4 gas

A cross section set for scattering Ne+ ions in CF4 gas is assessed by using available experimental data for charge transfer cross sections. Calculated cross sections can be used to obtain transport parameters, specially characteristic energy and reduced mobilty for low and moderate reduced electric fields E/N (E-electric field strength; N-gas density) and accounting for the non-conservative collisions. These data are needed for modelling in numerous applications of technologically importance.

Collisions of proton or highly charged ion–atom in a strong magnetic field and dense quantum plasmas

Magneto inertial fusion driven by heavy ions beam (HIB) is a very attractive potential approach for the nuclear energy system. One of the key issues is to investigate the interaction process of the HIB-target considering the condition of plasma screening and strong magnetic field background. In this paper, the influence of the external magnetic field and the plasma screening was investigated by simplifying the process of beam bombarding into a two-body collision between the energetic ions and target atoms. The classical-trajectory Monte Carlo method was accommodated by modifying the Hamiltonian in the collision system, where the effects of plasma screening and the account for the strong magnetic field background were considered. The total cross sections of single electron ionization and charge transfer of the projectile (H+, He2+, Xe32+, Bi31+, U34+)–atom (H, He) collisions are computed. The results indicated that the magnetic field effect becomes more obvious in the low energy regime of the projectiles. With the increase in energy, the change of total cross sections and angular differential cross sections gradually disappears. In the dense quantum plasmas, plasma screening presents very different effects for protons and heavy ion projectiles. This work may pave the way for extending the study to other collision systems calculations.

Model of charge transfer collisions between C60 and slow ions.

A semiclassical model describing the charge transfer collisions of C60 fullerene with different slow ions has been developed to analyze available observations. These data reveal multiple Breit-Wigner-like peaks in the cross sections, with subsequent peaks of reactive cross sections decreasing in magnitude. Calculations of charge transfer probabilities, quasi-resonant cross sections, and cross sections for reactive collisions have been performed using semiempirical interaction potentials between fullerenes and ion projectiles. All computations have been carried out with realistic wave functions for C60's valence electrons derived from the simplified jellium model. The quality of these electron wave functions has been successfully verified by comparing theoretical calculations and experimental data on the small angle cross sections of resonant C60+C60 + collisions. Using the semiempirical potentials to describe resonant scattering phenomena in C60 collisions with ions and Landau-Zener charge transfer theory, we calculated theoretical cross sections for various C60 charge transfer and fragmentation reactions which agree with experiments.

Final-state-resolved mutual neutralization in I+ - I− collisions

We have studied the mutual neutralization reaction of atomic iodine ions (i.e., ${\mathrm{I}}^{+}+{\mathrm{I}}^{\ensuremath{-}}\ensuremath{\rightarrow}\text{I}+\text{I}$) in a cryogenic double electrostatic ion-beam storage-ring apparatus. Our results show that the reaction forms iodine atoms either in the ground-state configuration ($\mathrm{I}(5{p}^{5}\phantom{\rule{0.16em}{0ex}}{}^{2}{P}^{\ensuremath{\circ}})$, \ensuremath{\sim}40%) or with one atom in an electronically excited state ($\mathrm{I}(6s{\phantom{\rule{0.16em}{0ex}}}^{2}[2]$), $\ensuremath{\sim}60%$), with no significant variation over the branching ratios in the studied collision-energy range (0.1--0.8 eV). We estimate the total charge-transfer cross section to be of the order of ${10}^{\ensuremath{-}13}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{2}$ at 0.1 eV collision energy. Ab initio relativistic electronic structure calculations of the potential-energy curves of ${\mathrm{I}}_{2}$ suggest that the reaction takes place at short internuclear distances. The results are discussed in view of their importance for applications in electric thrusters.

Measurements of charge transfer and target-electron-loss cross sections forH+,D+, andHe+impact on lithium at low energies

Experimental cross section measurements for charge transfer and for electron loss from lithium via charge transfer and direct ionization are reported for collisions between light ions and lithium atoms. Beams of protons, deuterium ions, and helium ions intersect a lithium atom beam, and collision cross sections are inferred by measuring the number of lithium ions produced in the collisions. The collision energy ranges studied are 0.13--3.5 keV for proton impact, 0.13--0.83 keV for deuterium ion impact, and 0.28--3 keV for helium ion impact. At proton collision energies below 0.7 keV our charge transfer cross sections are in excellent agreement with the theoretical predictions, resolving a long-standing discrepancy between theory and experiment in this energy range. Cross sections for proton and for deuterium ion impact on lithium are measured to be identical when scaled to the same collision velocity, indicating that quantum effects are not significant in the energy range studied. Comparison between our cross section measurements for lithium electron loss due to helium ion impact and previously measured charge transfer cross sections show that ionization is a negligible process in ${\mathrm{He}}^{+}+\mathrm{Li}$ collisions at energies below 3 keV. Undulations in the measured collision cross sections at low energy are observed for both proton and helium ion impact and are compared to molecular orbital close-coupling calculations.

Charge transfer in collisions of H&lt;sup&gt;+&lt;/sup&gt;, Li&lt;sup&gt;3+&lt;/sup&gt;, Be&lt;sup&gt;4+&lt;/sup&gt; and O&lt;sup&gt;7+&lt;/sup&gt; 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&lt;sup&gt;3+&lt;/sup&gt;, Be&lt;sup&gt;4+&lt;/sup&gt; and O&lt;sup&gt;7+&lt;/sup&gt;) 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.

Charge transfer and excitation processes in low energy collisions of He+ ions with Li atoms

Electron capture between solar wind ions and neutral species has contributed to the understanding of X-ray production from solar system bodies. The charge transfer and excitation processes in solar wind ions of He+(1s) colliding with Li(1s 22s) atoms are studied by utilizing the full quantum-mechanical molecular-orbital close-coupling (QMOCC) method with impact energies of 0.003–2 keV amu−1. Comparisons of cross sections from single- and multi-configurational calculations for a self-consistent field (SCF and MCSCF) process are carried out. Results show that the dominant reaction channels are He(1s2l 1,3L) + Li+(1s 2 1S). Good consistency is found among present total and state-selective charge transfer and excitation cross sections with other theoretical and experimental data in the same energy region. Due to the differences between coupling matrix elements in high-energy states, the charge transfer cross sections calculated from SCF and MCSCF split slightly as E > 0.4 keV amu−1. Weak Stueckelberg oscillations for charge transfer appear in the present work. In addition, the differences of cross sections for electron excitation to Li(1s 22p) in the singlet/triplet molecular states with He+(1s) are much smaller than those of charge transfer processes because of the similar energy gaps from Li(1s 22p) to the ground state in singlet/triplet states in the large R region.

Total and subshell-selective charge exchange cross sections in collisions of highly-charged Ne ions with atomic hydrogen

Total and subshell-selective charge exchange cross sections and the corresponding reaction rate coefficients for Neq+-H (q=6–10) collision systems are calculated by utilizing the two-center atomic orbital close-coupling (TC-AOCC) method at impact energy between 0.1 keV/u and 500 keV/u. Good agreements with the experimental and other theoretical results are found for the total charge transfer cross section in the considered energy range and for those of the dominant charge-exchange channels for the higher collision energies. These charge exchange cross section data are useful for understanding and modeling the X-ray emission in environments where charge transfer is an important mechanism generating this emission.

Single and double charge transfer in the Ne2++He collision within time-dependent density-functional theory

We calculate the charge-transfer cross sections for the ${\mathrm{Ne}}^{2+}+\text{He}$ collision. To this end, we employ Ehrenfest molecular dynamics with time-dependent density-functional theory. The active electrons of the projectile are handled by applying an initial velocity to the Kohn-Sham orbitals via a Galilean boost. The dynamical calculations are performed in an inverse collision framework---the reference frame considers ${\mathrm{Ne}}^{2+}$ to be initially at rest, which ensures numerically converged final-time scattering states. The charge-transfer probabilities are extracted by extending the particle number projection technique to be able to handle the degenerate ${\mathrm{Ne}}^{2+}$ ion. Compared with experimental data available at 10--3000 keV, a fairly good agreement is found for the calculated single- and double-charge transfer cross sections, superior to other theoretical calculations for this ${\mathrm{Ne}}^{2+}+\text{He}$ collision. A time-resolved analysis of the charge-transfer probabilities finds that ionization to the continuum also takes place after the charge transfer has occurred. To account for it, the final scattering states should be followed for a long time, approximately 350 fs, until they stabilize.

Self-Reactions in the HBr+ (DBr+) + HBr System: A State-Selective Investigation of the Role of Rotation.

Self-reactions observed in the HBr+ (DBr+) + HBr system have been investigated using a guided ion-beam experiment under single-collision conditions. The reaction channels observed are proton transfer/hydrogen abstraction (PT/HA) in the case of HBr+ and deuteron transfer/hydrogen abstraction (DT/HA) and charge transfer (CT) in the case of DBr+. HBr+/DBr+ ions have been formed with rotational energies selected using the resonance-enhanced multiphoton ionization (REMPI) formation process. Cross sections have been measured as a function of the rotational energy of the ion, Erot, and of the center-of-mass collision energy, Ecm. In the region of low rotational energies, the cross section for both PT/HA and DT/HA decreases with increasing ion rotation. In this region, the cross section for CT increases with increasing ion rotation. For higher rotational energies, the cross section increases with increasing ion rotation for PT/HA and less pronounced for DT/HA. The cross section for CT becomes independent of ion rotation for high rotational energies. Since all reaction channels are exothermic, all cross sections decrease with increasing Ecm.

The Rb + –Rb collision rate in the energy range of 103–104 K

We study both theoretically and experimentally the energy dependence of the low-energy Rb+–Rb total collision rate k ia in the energy range from 103 to 104 K. We calculate the integral elastic cross-section and the resonant charge-transfer cross-section by the quantum mechanical molecular orbital close-coupling method, and then obtain k ia for temperatures by averaging the cross-sections over a Maxwell–Boltzmann velocity distribution. The experiments are conducted in an ion–neutral hybrid trap, where the Rb+ ions are created by photo-ionization of the cold atoms in a magneto-optic trap (MOT) and accumulated in the linear Paul ion trap. The total ion–atom collision rate k ia is measured by monitoring the fluorescence reduction of the steady-state MOT atoms by sequentially introducing photo-ionization and ion–atom collisions. The ion–atom collision energy E col ≈ T i is modified by changing T i due to T i being more than six orders of magnitude larger than the T a of cold atoms. The temperature of ions T i is obtained by comparing the time-of-flight mass spectrometry of Rb+ from experimental results to that obtained by SIMION simulation. The equilibrium steady T i is modified by changing the initial root-mean-squared position of the ion cloud, and the k ia are measured with E col from 8000 to 16 000 K. Both the theoretical and experimental results show that k ia increases with E col. More specifically, the measured k ia increases rapidly with the enlargement of E col near 10 000 K. The theoretical calculation results show that k ia increases slowly with E col. The specific difference may be due to the influence of the ratio of excited states f e on the trend of k ia at different E col.

Acceptance-angle effects on the charge transfer and energy-loss cross sections for collisions of C4+ with atomic hydrogen

The charge-transfer process for collisions of ${\mathrm{C}}^{4+}$ with atomic hydrogen is studied theoretically and experimentally in this work. Our theoretical study is based on an electron-nuclear dynamics approach applied here for the state-to-state and total contributions to the electron-capture cross sections. Our theoretical results are complemented by experimental measurements of the absolute total cross section for collisions of ${\mathrm{C}}^{4+}$ with atomic hydrogen, which were carried out using an ion-atom merged-beams technique at relative collision energies of $0.122--2.756$ keV/u and performed with an improved apparatus at Oak Ridge National Laboratory. We find that the structure observed around a collision energy of 0.5 keV/u in the experimental results is due to the combined contributions of the $3\ensuremath{\ell}$ capture cross sections, the coupling of the electronic and nuclear dynamics, and the acceptance angle in the experimental configuration. We also report the ${\mathrm{C}}^{4+}$ kinetic energy loss and stopping cross section. We find that the ${\mathrm{C}}^{4+}$ gains energy for relative collision energies between 0.1 and 10 keV/u, with a maximum at $\ensuremath{\sim}1$ keV/u. Our theoretical study shows that, to compare to the merged-beams experimental results, one has to account for effects produced by the merged path length of the apparatus.

Line intensities for x-ray emission in Mg12+ collisions with H and He atoms

State selective charge transfer cross sections are calculated in Mg12+ collisions with H and He atoms at incident energies of 1.0, 3.0, and 5.0 keV amu−1 using a time-dependent lattice method. Total capture cross sections using the time-dependent lattice method are found to be larger than those previously calculated using the Landau–Zener method. Lyman line ratios are calculated using the time-dependent lattice capture cross sections and standard radiative transitions for both the H and He atoms. Lyman line probabilities are presented for both H and He. The strongest probability for both H and He is the Lyman α emission, but relatively strong probabilities for He at an incident energy of 1.0 keV amu−1 are found for Lyman β, γ, δ, and ϵ emission.

Influence of collisions on ion dynamics in the inner comae of four comets.

Collisions between cometary neutrals in the inner coma of a comet and cometary ions that have been picked up into the solar wind flow and return to the coma lead to the formation of a broad inner boundary known as a collisionopause. This boundary is produced by a combination of charge transfer and chemical reactions, both of which are important at the location of the collisionopause boundary. Four spacecraft measured ion densities and velocities in the inner region of comets, exploring the part of the coma where an ion-neutral collisionopause boundary is expected to form. The aims are to determine the dominant physics behind the formation of the ion-neutral collisionopause and to evaluate where this boundary has been observed by spacecraft. We evaluated observations from three spacecraft at four different comets to determine if a collisionopause boundary was observed based on the reported ion velocities. We compared the measured location of the ion-neutral collisionopause with measurements of the collision cross sections to evaluate whether chemistry or charge exchange are more important at the location where the collisionopause is observed. Based on measurements of the cross sections for charge transfer and for chemical reactions, the boundary observed by Rosetta appears to be the location where chemistry becomes the more probable result of a collision between H2O and H2O+ than charge exchange. Comparisons with ion observations made by Deep Space 1 at 19P/Borrelly and Giotto at 1P/Halley and 26P/Grigg-Skjellerup show that similar boundaries were observed at 19P/Borrelly and 1P/Halley. The ion composition measurements made by Giotto at Halley confirm that chemistry becomes more important inside of this boundary and that electron-ion dissociative recombination is a driver for the reported ion pileup boundary.

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.