Single-electron Capture Processes Research Articles

Direct Evidence of Breakdown of Spin Statistics in Ion-Atom Charge Exchange Collisions.

Recent experimental studies have questioned the validity of spin statistics assumptions, particularly in charge exchange processes occurring in atomic MeV collisions. Here, we study spin-resolved single electron capture processes in collisions between C^{3+} ions and helium within an energy range of 1.25-400 keV/u. Using high resolution reaction microscope and multielectronic theoretical approaches, we directly measure and calculate the true population information of the C^{2+}(1s^{2}2s2p ^{1,3}P) states at the time of electron capture, overcoming the previous experimental and theoretical difficulties. At the level of integral and scattering angle differential cross sections, our results demonstrate the breakdown of pure spin statistics arguments, especially at high impact energies where they are traditionally expected to be valid. These novel findings and conclusions raise intriguing questions both in the understanding of the electronic dynamics during such fast collisional processes and in exploring quantum manipulation of atomic and molecular reactivity.

Observing quantum matter-wave diffraction in the energetic He2+-He collisions

We present an experiment combined with numerical simulations to study single and double electron capture processes induced during the collisions between the He target and the He2+ projectile in the energy range 2–20 keV/u. According to our experimental observations, measuring the angular distribution of the scattered projectile He2+ reveals a clear oscillatory structure. The latter records an imprint of the collision dynamics, specifically, a signature of the matter-wave scattering and the internal electronic structure of the collisional system He2+-He. We found that this feature is sensitive to the incident projectile's energy and the nature of the involved process. With the help of four-body semiclassical close coupling and classical trajectory Monte Carlo methods, we show that the observed structure is of a quantum nature. This is further supported by a simple mathematical model based on Fraunhofer-type diffraction of light, to which the oscillations are attributed. Our findings thus provide insights into the role of quantum phenomena in characterizing collision dynamics. Published by the American Physical Society 2024

State-selective charge exchange cross sections in collisions of highly-charged sulfur ions with helium and molecular hydrogen

The state-selective cross section data are useful for understanding and modeling the x-ray emission in celestial observations. In the present work, using the cold target recoil ion momentum spectroscopy, for the first time we investigated the state-selective single electron capture processes for S q+–He and H2 (q = 11–15) collision systems at an impact energy of q × 20 keV and obtained the relative state-selective cross sections. The results indicate that only a few principal quantum states of the projectile energy level are populated in a single electron capture process. In particular, the increase of the projectile charge state leads to the population of the states with higher principal quantum numbers. It is also shown that the experimental averaged n-shell populations are reproduced well by the over-barrier model. The database is openly available in Science Data Bank at 10.57760/sciencedb.j00113.00091.

Аномальное сечение диссоциативного захвата при взаимодействии однозарядных ионов с молекулами оснований нуклеиновых кислот

The single electron capture process from adenine molecules by H+, 3He+, N+, O+, and Ne+ ions with an energy of 6.3 keV has been studied. It was found that when one electron is captured by He+ and Ne+ ions, the dissociative capture process is dominant, which is fundamentally different from the processes of capture by singly charged ions of atoms with a lower ionization potential and multiply charged ions, as well as from the process of electron impact ionization. This effect is qualitatively explained within the framework of a quasimolecular model.

Anomalous Dissociative Capture Cross Section at the Interaction of Singly Charged Ions with Molecules of Nucleic Acid Bases

The single electron capture process from adenine molecules by H+, 3He+, N+, O+, and Ne+ ions with an energy of 6.3 keV has been studied. It was found that when one electron is captured by He+ and Ne+ ions, the dissociative capture process is dominant, which is fundamentally different from the processes of capture by singly charged ions of atoms with a lower ionization potential and multiply charged ions, as well as from the process of electron impact ionization. This effect is qualitatively explained within the framework of a quasimolecular model. Keywords: dissociative capture, nucleic acid bases, fragmentation, Landau-Zener model.

Cross sections for electron capture process of biological molecules by proton and α-particle impact

SynopsisDistorted wave model has been used to predict total cross sections (TCSs) of single electron capture process for several ion-biological molecule collisional systems in the intermediate and high energy regime. TCSs are calculated in the Independent Electron Model (IEM) In this work, we studied with water, nitrogen, oxygen, methane, adenine or cytosine targets impacted by protons and α-particles with kinetic energy ranging from 25 keV/amu to 10,000 keV/amu. It is observed that the present investigation produces results which are in good agreement with experimental observations for all cases.

State-selective single electron capture in slow O 6+-He collisions

SynopsisThe state-selective single-electron capture processes were measured by cold-target recoil-ion momentum spectroscopy in O 6+-He collisions between 18keV and 96keV. From the longitudinal momentum spectrum of recoil ions, the single electron captured into n=3 states of projectile ion is predominant and n=4 states have a small contribution. The oscillation structure of angular differential cross sections was observed. The present experimental results are compared with other experiments at low energy and the predictions of the molecular Columbic barrier model.

Electron capture, excitation and ionization in H+–Be+ collisions

State-selective single electron capture, excitation and ionization processes in proton collisions with Be+(1s22s) and Be+(1s22p) ions are studied by using the quantum-mechanical molecular orbital (QMOCC) and the two-center atomic orbital close-coupling (TC-AOCC) methods in the combined energy range 0.01–300 keV/u. In the QMOCC calculations six 2Σ+ and three 2Π states are included in the expansion basis. The TC-AOCC expansion basis includes 35 discrete states centered on the proton and 182 states on Be2+ out of which 88 are continuum pseudostates. The atomic states on Be2+ are generated by a one-electron model potential for the electron interaction with Be2+ ion core that reproduces the exact energies with accuracy better than 0.5%. Total and state-selective electron capture and excitation cross sections to final states with n ≤ 4 are presented. The sum of the cross sections for the transitions to continuum pseudostates (ionization cross section) is also reported for both initial target states. The physical mechanisms involved in the dynamics of considered processes are discussed in detail at both low and high energies. The reported cross sections should be useful in the kinetic modeling and diagnostics of edge plasmas in present magnetic fusion experiments, as well as in the current ITER design, in which beryllium is used as first wall material.

Electron capture and excitation in He+–He(1s2s1,3S) collisions

State-selective single-electron capture and excitation processes in collisions of He+ ions with metastable He(1s2s1,3S) excited atoms are investigated by using the two-center atomic orbital close-coupling method in the energy range 0.5–100 keV/u. The expansion basis includes all the states with n ≤ 9, lmax = 4 on the projectile and all the states with n ≤ 5, lmax = 4 on the target, when treating the electron capture and vice versa, when treating the ionization. One-electron model potentials for the singlet and triplet series of states have been used that reproduce the exact energies within an accuracy of 1%–2%. Spin-resolved electron capture and excitation cross sections to nl final states with n ≤ 4 are presented. The origin of differences in the magnitude and energy behavior of spin-resolved cross sections is discussed. The reported cross section results should be useful in the studies of properties of various laboratory and astrophysical helium plasmas.

The influence of pseudo states on the single-electron capture processes in low-energy collisions of N5+ with He

The nonradiative single-electron capture (SEC) processes for N5+(1s2) colliding with He atoms are investigated by using the quantum-mechanical molecular-orbital close-coupling method. Total and state-selective SEC cross sections are obtained in the energy range of 3–1700 eV amu−1, for which the ab initio potential curves and nonadiabatic coupling matrix elements (involving physical and pseudo states) are computed by using the multireference single- and double-excitation configuration-interaction method. It is found that the pseudo states are also important to obtain the convergent results, especially in the low-energy region. Moreover, our results indicate that the n = 3 states are the dominant states in the considered energy region, and that the electron translation factor effects become important when the energy is above 1 keV amu−1. In addition, it is observed that the total and state-selective cross sections for SEC are in general agreement with all available experimental data.

Quantum mechanical potentials and inactive electron effects in charge exchange collisions

Scattering angle differential cross sections for the He+ + He single electron capture process are studied using plane wave Born approximation models for projectile energies between 180 keV and 1.89 MeV. Within this simplistic framework, we study the effects of the frozen core approximation by performing a full 5-Body calculation that explicitly includes all particles in the collision and comparing it with a single bound state model that neglects the bound electron in the projectile and a double bound state model that neglects the inactive electron in the target atom. Results are compared with experiment and we show that inclusion of the inactive electron in the perturbation potential is more important than inclusion in the wave functions. We also introduce a semi-quantum mechanical (SQM) perturbation potential that treats the atomic electrons as a quantum mechanical electron cloud rather than point particles. The SQM perturbation removes the deep, unphysical minimum that exists in cross sections calculated with Born-type models, but also has the effect of greatly reducing the magnitude of the small scattering angle cross sections.

Classical simulation of differential single charge transfer in fast proton-helium collisions

Three-body classical trajectory Monte Carlo method is employed to simulate the differential single electron capture process in fast proton-helium collisions. For the considered collisional system, by means of an independent particle model, both electron capture and electron excitation probabilities are evaluated in terms of the classical impact parameter and the related discussions are presented. The method is also applied to calculate the projectile-angular distribution of the cross sections in energy range of 50–630 keV. The obtained results are compared to the available precise data due to the cold-target recoil ion momentum spectroscopy and good overall agreement found with these experimental data. Also, within a classical-trajectory framework, the correlation between the impact parameter and the projectile scattering angle is examined through the simulation of the collision process.

Role of the recoil ion in single-electron capture and single-ionization processes for collisions of protons with He and Ar atoms

In this work the single-electron capture and single-ionization processes are studied for proton collisions with He and Ar atoms at impact energies in the range 25--100 keV. Classical trajectory Monte Carlo simulations are benchmarked against experimental data obtained at the reaction microscope in Bariloche, Argentina, which employs the cold target recoil-ion momentum spectroscopy technique. Special emphasis is placed on describing the momentum transfer to the recoil ion for these collision systems.

One-electron capture from K shell of atomic targets by proton impact

A first-order four-body perturbation theory is developed to calculate both the differential and integral cross-sections for [Formula: see text]-shell charge exchange from multi-electron atomic targets to the [Formula: see text] bound state of the fast proton projectiles. The correct boundary conditions are incorporated in the formalism. The model is applied to the single electron capture process from carbon, nitrogen, oxygen, neon and argon atoms for which the experimental data are available. The results are compared with their corresponding experimental values and also with those obtained from three-body version of the theory. A comparison is also made between the present predicted cross-sections and those obtained from other theories. Comparisons show that the suggested approximation is in reasonable agreement with the experimental data and is compatible with the other theories.

Single electron ionization and electron capture cross sections for (C6+, H2O) interaction within the Classical Trajectory Monte Carlo (CTMC) approach

In this work, we present a derivation of cross sections for single ionization and electron capture processes within the Classical Trajectory Monte Carlo (CTMC) approach. Specifically, we have used a potential stemming from an ab initio calculation in Green et al.’s framework to describe the dynamics of the water molecule system. Proposing a modified version of the Classical Over-Barrier (COB) potential, we have found that a cut-off of roughly 28a.u. on the initial distance of the projectile produced a reasonable accuracy. A global agreement has been obtained in our calculations compared to experimental and other theoretical results for C6+ ion energies ranging from 10keV/u to 10MeV/u.

Eikonal approximation for single-electron capture from hydrogen molecules by proton impact

A four-body eikonal approximation is developed to study the single-electron capture process in collision of the fast protons with the hydrogen molecular targets. For a fixed orientation of the molecular axis, the double-differential cross sections are evaluated for electron capture. Interference patterns originated from the two atomic centres are obtained for different orientations of the molecular target. Equal-weighted average of the differential capture cross sections over all the possible orientations of the molecular target is calculated for various impact energies. Angular distributions of the differential cross sections for single charge transfer at various impact energies as a function of the angle between the axis of the molecule and the incident beam direction arecalculated and compared with their corresponding experimental values as well as the results obtained from other theories. Integrated cross sections are calculated and compared with available experimental data and other theoretical calculations.

Interatomic Coulombic decay as a new source of low energy electrons in slow ion-dimer collisions.

We provide the experimental evidence that the single electron capture process in slow collisions between O^{3+} ions and neon dimer targets leads to an unexpected production of low-energy electrons. This production results from the interatomic Coulombic decay process, subsequent to inner-shell single electron capture from one site of the neon dimer. Although pure one-electron capture from the inner shell is expected to be negligible in the low collision energy regime investigated here, the electron production due to this process overtakes by 1 order of magnitude the emission of Auger electrons by the scattered projectiles after double-electron capture. This feature is specific to low charge states of the projectile: similar studies with Xe^{20+} and Ar^{9+} projectiles show no evidence of inner-shell single-electron capture. The dependence of the process on the projectile charge state is interpreted using simple calculations based on the classical over the barrier model.

Electron capture and deprotonation processes observed in collisions between Xe8+and multiply protonated cytochrome-C

Electron-transfer processes in interaction between highly charged ions and multiply protonated proteins have been studied. Collisions between Xe${}^{8+}$ at 96 keV and protonated cytochrome-C at selected charge state ($q$ from 15+ to 19+) result in mass spectra composed mainly of intact molecular ions. From the spectra, single and double electron capture processes by Xe${}^{8+}$ from the protonated molecular ions were identified and the relative cross sections were measured. An unexpected process, the deprotonation process, was also observed. It is tentatively attributed to the loss of a proton induced by the strong electric field carried by the projectile ion in long-distance collisions. Upon charge variation of the molecular target from 15 to 19, the single and double electron capture cross sections remain nearly constant, while the relative cross section of the deprotonation process increases dramatically from 0.8% (\ifmmode\pm\else\textpm\fi{}0.1%) to 17% (\ifmmode\pm\else\textpm\fi{}1%). This strong charge dependency is explained by the decrease of the proton affinities with the charge. This proton removal process has not been observed previously. It seems to be specific to the long-distance Coulomb interactions between protons bound along the protein chain and the highly charged atomic ions.

Proton-induced single electron capture on DNA/RNA bases

In this work, we report total cross sections for the single electron capture process induced on DNA/RNA bases by high-energy protons. The calculations are performed within both the continuum distorted wave and the continuum distorted wave-eikonal initial state approximations. The biological targets are described within the framework of self-consistent methods based on the complete neglect of differential overlap model whose accuracy has first been checked for simpler bio-molecules such as water vapour. Furthermore, the multi-electronic problem investigated here is reduced to a mono-electronic one using a version of the independent electron approximation. Finally, the obtained theoretical predictions are confronted with the scarcely available experimental results.

Single- and double-electron capture processes in low-energy collisions of N3+with He

Single-electron capture (SEC) and double-electron capture (DEC) processes in collisions of ground state N{sup 3+} (2s{sup 2} {sup 1}S) ions with He are investigated by using the quantum-mechanical molecular-orbital close-coupling (QMOCC) method. The ab initio adiabatic potentials, radial and rotational coupling matrices utilized in QMOCC calculations, are obtained from the multireference single- and double-excitation configuration interaction approach. Total and state-selective SEC and DEC cross sections are presented in the low-energy range from 0.1 eV to 15 keV (i.e., 0.007 eV/u -1.07 keV/u) and rate coefficients in the temperature range from 10{sup 4} to 10{sup 7} K. Our results indicate that the SEC dominates the charge-transfer process in the considered energy region of this collision system and the SEC cross sections are nearly constant in the relatively high-collision energy region, while the DEC cross sections are about 2 orders of magnitude smaller. It is found that, for the SEC processes, in the dominant mechanisms, electrons are captured to exoergic channels N{sup 2+} (2s2p{sup 2} {sup 2}D,{sup 2}S), and for the DEC processes, they are captured to N{sup +} (2s{sup 2}2p{sup 2} {sup 1}D,{sup 1}S). Our calculations also reveal that rotational couplings become important at E > 10 eV/u for SEC andmore » E > 200 eV/u for DEC processes.« less