Charge Transfer Processes In Collisions Research Articles

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.

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.

Single charge exchange in collision of fast protons with hydrogen molecules

Single charge transfer process in collision of energetic protons with molecular hydrogens is theoretically studied using a first-order two-effective-center Born approximation. The correct boundary conditions are incorporated in the formalism and the Hartree–Fock molecular wave function for molecular targets and the residual ions are used to calculate the transition amplitude. The interference patterns in the capture differential cross-sections (DCSs) for a given fixed orientation of the molecule, due to the scattering from the two-atomic centers in the molecular targets, are examined. The dependence of the DCSs upon the angle between the molecular axis and the direction of the incident velocity is theoretically investigated. Both average differential and integral cross-sections are calculated. The obtained results are compared with the available experimental data.

Four-body charge transfer processes in collisions of bare projectile ions with helium atoms

Single-electron capture by a bare ion from a helium atom at intermediate and high energies in the framework of four-body distorted wave (DW-4B) approximation in both prior and post form has been considered. In the entrance channel, the initial bound state wave function is distorted by the incoming projectile ion, and the corresponding distortion is related to the Coulomb continuum states of the active electron and the residual target ion in the field of the projectile ion respectively. Continuum states of the active electron and the projectile ion in the field of the residual target ion are also included in the exit channel. It may be mentioned that the effect of dynamic electron correlation is explicitly taken into account through the complete perturbation potential. The total single-electron capture cross sections are obtained by summing over all contributions up to n = 3 shells and sub-shells respectively. In addition, the differential cross sections for alpha particle–helium collision are calculated at impact energies of 60, 150, 300, 450, and 630 keV amu−1, respectively. The cross sections exhibit a monotonically decreasing angular dependence, with clear peak structures around 0.1 to 0.2 mrad being found at low impact energies. The current theoretical results, both in prior and post forms of the transition amplitude for symmetric and asymmetric collision, are compared with the available theoretical and experimental results. Current computed results have been found to be satisfactory in comparison with other theoretical and experimental findings.

Differential Single-Capture Cross Sections for Fast Alpha–Helium Collisions

A four-body theoretical study of the single charge transfer process in collision of energetic alpha ions with helium atoms in their ground states is presented. The model utilizes the Coulomb–Born distorted wave approximation with correct boundary conditions to calculate the single-electron capture differential and integral cross sections. The influence of the dynamic and static electron correlations on the capture probability is investigated. The results of the calculations are compared with the recent experimental measurements for differential cross sections and with the other theoretical manipulations. The results for scattering at extreme forward angles are in good agreement with the experimental measurements, but in other scattering angles the agreement is poor. However, the present four-body results for integral cross sections are in excellent agreement with the experimental data.

Charge transfer of slow highly charged xenon ions in collisions with magnesium atoms

We report an experimental study of the charge-transfer process in collisions of Xe${}^{q+}$ ions ($16\ensuremath{\le}q\ensuremath{\le}20$) with magnesium atoms at an energy of 5.5$q$ keV. With charge-selective and time-coincidence techniques, we separated the pure capture and capture accompanied by transfer-ionization processes. The experimental data indicate that the magnesium target is around two times more likely to lose two electrons than one in the collision. This finding is very different compared to the calculation based on the extended classic over-the-barrier model. The Xe${}^{q+}$-Mg collision also behaves very differently from ``traditional'' collisions between highly charged ions and noble gases. We suggest a one-step dielectronic mechanism for the capture process. The data also show that autoionization dominates the relaxation process after the capture, and fluctuation of the autoionization fraction versus the projectile charge state indicates that for the relaxation processes, the projectile core structure plays a more important role than the detailed characteristics of the projectile states where the target electrons are initially captured.

Ab initiostudy of charge-transfer dynamics in collisions of C2+ions with hydrogen chloride

Ab initio quantum chemistry molecular calculations followed by a semiclassical dynamical treatment in the keV collision energy range have been developed for the study of the charge-transfer process in collisions of ${\mathrm{C}}^{2+}$ ions with hydrogen chloride. The mechanism has been investigated in detail in connection with avoided crossings between states involved in the reaction. A simple mechanism driven by a strong nonadiabatic coupling matrix element has been pointed out for this process. A comparative analysis with the halogen fluoride target corresponding to a similar electronic configuration shows a quite different charge-transfer mechanism leading to a very different behavior of the cross sections. Such behavior may be correlated to specific nonadiabatic interactions observed in these collision systems.

Charge transfer dynamics of carbon ions with uracil and halouracil targets at low collision energies

A theoretical treatment is derived at low-energies for the single charge transfer process in collision of C4+ ions with uracil and halouracil targets using ab-initio molecular methods. The radiosensitivity properties of halouracils and the anisotropy of the process are analyzed and compared to previous calculations at keV energies.

Charge transfer rate coefficients in collision of C2+ ions with CO and N2 molecular targets

An ab initio molecular treatment of the charge transfer process in collision of C2+ ions with the isoelectronic CO and N2 targets has been performed. The corresponding rate coefficients in the 400–50000K temperature range are compared to recent measurements of Gao and Kwong at Tequiv=1.17×104K. A global satisfactory agreement is observed and some general behaviour may be deduced.

Polarization Degrees for 3p 2P3/2–3s 2S1/2 Transition of C3+ (1s23p) Produced in Collisions of C4+ with He and H2

Charge transfer processes in collisions of C 4+ with ground-state He and H 2 are investigated using the two-center atomic orbital close-coupling method. Total and state-selective single-electron capture cross sections are obtained for collision energies between 1 and 7 keV/amu, and comparison with available experimental data reveals a good agreement. The polarization degrees for the 3 p 2 P 3/2 –3 s 2 S 1/2 transition of C 3+ (3 p 2 P 3/2 ) are calculated from the magnetic substate-selective cross sections, in which the cascade effects are also included. The polarization degree in the C 4+ +H 2 collisions varies smoothly in the considered energy region and generally agrees with the experimental measurements, while if appears minimum at a collision energy of 2 keV/u for C 4+ +He collisions. By studying the time-evolution of electron capture dynamics in the two systems we have found that the polarization difference between the two systems is caused mainly by interactions at relatively small internuclear dis...

Charge transfer processes in collisions of slow highly charged ions with polar molecules CO and C3H8

Charge transfer processes resulting from low energy collisions of polar molecules CO and C3H8 with highly charged Be2+ and B2+ ions have been investigated experimentally and theoretically. The potential for the one–electron exchange interaction between a polar molecule and highly charged atomic ion were obtained in closed analytical form in the framework of a semiclassical approach. Obtained results were used for close–coupling calculations of total cross sections for single electron transfer in the studied reactions.

Calculations of state-selective differential cross sections for charge transfer in collisions between O3+ and H2

The non-dissociative charge-transfer processes in collisions between O3+ and H2 are investigated by using the quantum-mechanical molecular-orbital coupled-channel (QMOCC) method. The adiabatic potentials and radial coupling matrix elements utilized in the QMOCC calculations are obtained with the spin-coupled valence-bond approach. Electronic and vibrational state-selective differential cross sections are presented for projectile energies of 0.1, 1.0 and 10.0 eV/u in the H2 orientation angles of 45° and 89°. The electronic and the vibrational state-selective differential cross sections show similar behaviours: they decrease as the scattering angle increases, and beyond a specific angle the oscillating structures appear. Moreover, it is also found that the vibrational state-selective differential cross sections are strongly orientation-dependent, which provides a possibility to determine the orientations of molecule H2 by identifying the vibrational state-selective differential scattering processes.

A simple theoretical approach of charge transfer processes in collisions of atomic ions with polar targets

Charge transfer processes resulting from low energy collisions of the molecules CO and C 3 H 8 with Be 2 + and B 2 + ions have been experimentally investigated. Theoretical study based on a semiclassical analysis of the wave function of tunneling electron was also carried out. The potential for the one-electron exchange interaction between a non-central target (polar molecule) and projectile (atomic ion) were obtained in closed analytical form. Our theoretical calculations are found to be in fairly good agreement with the measured cross sections and with the data of other authors.

Vibrational effect on charge-transfer processes in collisions ofH+andO+ions withC2H4molecules at energies below10keV∕u

Charge-transfer processes in collisions of ${\mathrm{H}}^{+}$ and ${\mathrm{O}}^{+}$ ions with ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ molecules have been studied in joint theoretical and experimental approaches for collision energies below $10\phantom{\rule{0.3em}{0ex}}\mathrm{keV}∕\mathrm{u}$. Since H and O atoms possess nearly identical ionization potentials, these two ions are expected to share similar dynamics and hence to have similar cross-section values for charge transfer, at least for the energy region above $100\phantom{\rule{0.3em}{0ex}}\mathrm{eV}$. In the present experiments, these cross sections of ${\mathrm{H}}^{+}$ and ${\mathrm{O}}^{+}$ ions have been derived by the initial growth rate method in the energy range of $0.20\phantom{\rule{0.3em}{0ex}}\text{to}\phantom{\rule{0.3em}{0ex}}3.6\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, while the present theoretical study has been carried out for ${\mathrm{H}}^{+}$ impact only. We have examined the vibrational effect on charge transfer, especially the ``temperature effect'' of the initial vibrational states of the ${\mathrm{C}}_{2}{\mathrm{H}}_{4}$ molecule as well as the final vibrational states of the product ${\mathrm{C}}_{2}{\mathrm{H}}_{4}^{+}$ molecular ion. Indeed, a strong influence of the initial vibrational states on charge transfer has been observed since these vibrationally excited initial and final states force the process to be more nearly resonant, thus making charge transfer more efficient. This vibrational effect, and perhaps rotational effect as well, that makes the collision process more nearly resonant has not been well investigated previously, and thus the present study is expected to shed much light on this effect in general.

Elastic and inelastic processes inH++C2H6collisions below the10−keVregime

Charge-transfer processes in collisions of ${\mathrm{H}}^{+}$ ions with ${\mathrm{C}}_{2}{\mathrm{H}}_{6}$ molecules are investigated theoretically below $10\text{\ensuremath{-}}\mathrm{keV}$ collision energies within a molecular representation. Converged total as well as differential cross sections are obtained in this energy range within a discrete basis of electronic states computed by ab inito methods. The present collision system suggests that the combination of the Demkov-type and Landau-Zener--type mechanisms primarily governs the scattering dynamics for the flux exit from the initial channel. The present charge-transfer cross sections determined are found to agree very well with all available experimental data below a few keV, but begin to deviate above $3\phantom{\rule{0.3em}{0ex}}\mathrm{keV}$, in which the present results slowly decrease, while measurements stay nearly constant. From the study of the electronic state calculation, we provide some information on fragmented species, which should help shed some light on the fragmentation mechanism and process of ${\mathrm{C}}_{2}{\mathrm{H}}_{6}^{+}$ ions produced after charge transfer. In addition, the vibrational effect of the initial target to charge transfer is examined.

Electronic excitation and charge transfer processes in collisions between Mg(3(1)S0) atoms and Rb+(1S0) ions in the 0.07-4.00 keV energy range.

Inelastic collision processes between neutral Mg atoms and Rb(+) ions, both in their ground states, have been studied by means of a crossed molecular beam technique measuring the decay fluorescence of the excited species formed. Emissions corresponding to Mg(3 (1)P(1)), Mg(3 (3)D(3,2,1)), and Mg(4 (3)S(1)), formed by direct target excitation, Rb(5 (2)P(3/2,1/2)), Rb(6 (2)P(3/2,1/2)) produced by electron capture and also the phosphorescent emission due to decay of Mg(3 (3)P(1)), have been detected and the corresponding absolute cross-section values measured both as total values and resolved into their J states. No polarization measurements could be made. Ab initio calculations using pseudopotentials have been performed and from these a manifold of adiabatic energy curves correlating with the different entry and exit channels have been obtained, allowing to propose a qualitative interpretation of the results, such as the shape of the cross section vs energy for different transitions and the oscillating nature of the branching ratios due to interference effects.

An experimental and theoretical study of electronic excitation and charge transfer processes in collisions between Cs( [formula omitted]) atoms and [formula omitted] ions in the 0.30–4.00 keV energy range

The collisional system Cs + Na + with both species in their ground electronic state, has been studied using the crossed molecular beams technique and measuring the resulting fluorescent decay. Emissions from electronically excited Cs( 6 2 P 3/2 ) and Cs( 7 2 P 1/2,3/2 ) atoms have been detected as well as those from excited Na( 3 2 P 1/2,3/2 ) and Na( 4 2 D 3/2,5/2 ) produced by electron transfer. Cross-section values in the 0.30–4.00 keV energy range have been measured for the four different collision channels as well as the branching ratio between the resolved J-states for Cs( 7 2 P 1/2,3/2 ). A qualitative interpretation for some of the measured emission cross-sections as a function of the collision energy has been performed using ab initio one-electron calculations done for the (CsNa) + quasimolecule. Hemiquantal dynamical calculations have been carried out at selected collision energies including radial and angular non-adiabatic couplings, comparing the calculated cross-sections with the experimental data.

Observation of the isomer effect on charge transfer from C3H4 molecules (allene and propyne) at keV energies.

The apparent isomer effect on charge transfer processes in collisions of the ground-state C+ ions with allene and propyne, which have the same C3H4 molecular chemical notation, has been observed experimentally in the collision energy from 0.2 to 4.5 keV. The difference in total charge transfer cross sections between the two isomers was found to be 32% at 0.2 keV, although it decreases to 10% at 4.5 keV. This difference is out of experimental uncertainty of 14.6% at least below the collision energy of 0.5 keV and should be regarded as a real isomer effect. Theoretical analysis based on a molecular orbital expansion method has also confirmed the experimental finding and provided the rationale.

Charge Transfer Processes in Collisions of Si4+ Ions with He Atoms at Intermediate Energies

Charge transfer in collisions of Si4+ ions with He atoms below 100 keV/u is studied by using a molecular orbital representation within both the semiclassical and quantal representations. Single transfer reactionSi4+ + He → Si3+ + He+ has been studied by a number of theoretical investigations. In addition to the reaction (1), the first semiclassical MOCC calculations are performed for the double transfer channelSi4+ + He → Si2+ + He2+Nine molecular states that connect both with single and double electron transfer processes are considered in the present model. Electronic states and corresponding couplings are determined by the multireference single- and double- excitation configuration interaction method. The present cross sections tie well with the earlier calculations of Stancil et al., Phys. Rev. A 55, 1064 (1997) at lower energies, but show a rather different magnitude from those of Bacchus-Montabonel and Ceyzeriat, Phys. Rev. A 58, 1162 (1998). The present rate constant is found to be significantly different from the experimental finding of Fang and Kwong, Phys. Rev. A 59, 342 (1996) at 4,600 K, and hence does not support the experiment.

Charge transfer inF2+ions colliding with He atoms below keV energies and its reverse process

Charge transfer processes in collisions of ${\mathrm{F}}^{2+}$ ions with He atoms were studied by using the molecular representation below 600 eV energies. The ground ${\mathrm{F}}^{2+}{(}^{4}S)$ state and one excited ${\mathrm{F}}^{2+}{(}^{2}D)$ state are considered as the initial channels in order to examine the effect of the excited state. We have also investigated charge transfer processes in the reverse ion-ion collisions. Cross sections for nonradiative charge transfer in the ground quartet state ${\mathrm{F}}^{2+}+\mathrm{H}\mathrm{e}$ collisions are found to increase gradually with the impact energy and reach around $2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{2}$ at 558 eV, while those from the doublet state also show a gradual increase and stay nearly constant with $1.2\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}16}{\mathrm{cm}}^{2}$ at the same energy. Cross sections for the reverse processes for both symmetries are found to be very small with a magnitude less than ${10}^{\ensuremath{-}18}{\mathrm{cm}}^{2}$ due to the strong Coulomb repulsion between two ions, but rapidly increase as the collision energy increases to reach $3\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}17}$ and ${10}^{\ensuremath{-}17}{\mathrm{cm}}^{2}$ for the quartet and doublet manifolds, respectively, at the highest energy studied.