One-electron capture and target-ionization in He +-neutral-atom collisions

Single- and double-electron loss cross sections of He, and single-electron capture and single-electron loss cross sections of He+ in collisions with H2, O2, CH4, CO and CO2 have been measured in the energy range 0.3-1.8 MeV. From the measured cross sections, charge-changing cross sections in collisions of He and He+ with C have been determined using the Bragg rule. Scaling is examined for electron capture cross sections.

To examine the influence of radiative transitions on the performance of nonequilibrium seed-free argon plasma magnetohydrodynamic (MHD) generators, we developed MHD numerical simulation technique with a collisional-radiative (C-R) model, where neutral argon atoms with 45 discrete effective electronic excitation levels, singly ionized argon ions, and free electrons were considered under a two-temperature, low-magnetic Reynolds number MHD approximation. Furthermore, a so-called escape factor was implemented in the C-R model to consider the effects of radiation trapping by ground-state neutral atoms. Using the developed technique, we computed the performance characteristics of a nonequilibrium seed-free argon plasma MHD generator. The numerical results showed that the generator performance gets higher as the escape factor decreases, i.e., the optical thickness for radiative transitions to the ground state of neutral atoms increases. The numerical results also suggested that when the escape factor is <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$1.0\times 10^{-3}$ </tex-math></inline-formula> or less, the generator performance is almost the same as that for the escape factor of 0 corresponding to completely optical-thick plasma case for the radiative transitions to the ground state of neutral atoms. Furthermore, realistic escape factor values for resonance lines in the power generation channel were evaluated using a well-known estimation model. Consequently, the estimated values had the order of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math notation="LaTeX">$10^{-4}$ </tex-math></inline-formula> . We also showed that under such relatively optically thick plasma cases, a collisional (C) model and a widely used global ionization-recombination rate model with no explicit consideration of electronically excited-state neutral atoms can reproduce the generator performance predicted by the C-R model with relatively good accuracy.

The motion-induced dephasing is a severe problem that limits the accuracy of a quantum control process by using external laser fields in neutral Rydberg atoms. This dephasing is a major issue that limits the realizable fidelity of a quantum entangling gate with neutral atoms when there is a {\it gap} time for the Rydberg atom to drift freely. We find that such a dephasing can be largely suppressed by using a transition in a `V'-type dual-rail configuration. The left~(right) arm of this `V' represents a transition to a Rydberg state $|r_{1(2)}\rangle$ with a Rabi frequency $\Omega e^{ikz}~(\Omega e^{-ikz})$, where $z$ is frozen without atomic drift, but changes linearly in each experimental cycle. Such a configuration is equivalent to a transition between the ground state and a hybrid and time-dependent Rydberg state with a Rabi frequency $\sqrt2\Omega$, such that there is no phase error whenever the state returns to the ground state. We study two applications of this method. First, it is possible to faithfully transfer the atomic state between a hyperfine ground state $|1\rangle$ and Rydberg states $|r_{1(2)}\rangle$ with no {\it gap} time between the excitation and deexcitation. Second, by adding infrared laser fields to induce transition between $|r_{1(2)}\rangle$ and a nearby Rydberg state $|r_3\rangle$ via a largely detuned low-lying intermediate state in the {\it gap} time, the atom can keep its internal state in the Rydberg level as well as adjust the population branching in $|r_{1(2)}\rangle$ during the {\it gap} time. This allows an almost perfect Rydberg deexcitation after the {\it gap} time, making it possible to recover a high fidelity in the Rydberg blockade gate. The theory paves the way for high-fidelity quantum control over neutral Rydberg atoms without cooling qubits to the motional ground states in optical traps.

Translational energy spectroscopy (TES) in the range 214–857 eV amu−1 has beenused to study the collision mechanisms involved in one-electron capture by slowhelium-like N5+ in both atomic and molecular hydrogen. In the case of N5+–H2 collisions,our measurements show that non-dissociative electron capture leading to the N4+ (n = 3) states isthe main product channel at the higher impact energies with smaller contributions to theN4+ (n = 4)states. While this has also been observed in previous studies based on photonemission spectroscopy (PES), there are substantial differences in both magnitude andenergy dependence between the TES and PES results. Theoretical predictions for n = 3 formationare also in poor accord with experiment. Unlike previous PES measurements, thepresent TES study has been able to identify the presence of dissociative one-electroncapture channels and two-electron autoionizing capture channels, both leading to N4+(n = 2)formation. Two-electron autoionizing electron capture is found to be the maincollision mechanism leading to N4+ ions at the lowest energies considered. Ourmeasurements of one-electron capture in N5+ + H(1s) are in excellent accord withprevious higher measurements based on PES and now provide a usefulextension to energies below 1 keV amu−1. In this case, only the N4+ (n = 4) and N4+(n = 3)product channels are observed, with contributions from the latter becominginsignificant at our lowest energies.

Theoretical investigations on charge transfer in collisions of fully stripped ions with neutral atoms are exhaustive [1–4]. In contrast, a few theoretical studies [5–10] have so far been made to study charge transfer cross sections in collisions of partially stripped ions with neutral atoms at intermediate and high-energy region. In this respect, the scenario of experimental investigations on partially and fully stripped ions is well balanced. In this connection, works of Goffe et al [11] and Phaneuf et al [12] are worthy of mentioning. Olson and Salop [5] have calculated charge transfer and ionization cross sections by fully and partially stripped ions of Bq+, Cq+, Nq+ and Oq+ (q≥3) from atomic hydrogen in the framework of Classical Trajectory Monte Carlo (CTMC) simulation method in intermediate and high energy region. They have treated the interaction of the active electron with projectile ion as coulombic with an effective charge determined from spectroscopic data. Eichler et al [6] have calculated charge transfer cross sections for the same processes except for Bq+ replaced by Liq+ (q ≥ 3) in OBK- approximation. They have treated the projectile ions as bare ions with asymptotic charge (q). However, they have multiplied the calculated cross sections by a reduction factor obtained from eikonal approximation and successively by Pauli blocking factor.

Two Wide-angle Imaging Neutral-atom Spectrometers (TWINS) is the first stereoscopic magnetospheric imager. TWINS is a NASA Explorer Mission of Opportunity performing simultaneous energetic neutral atom (ENA) imaging from two widely-separated Molniya orbits on two different spacecraft, and providing nearly continuous coverage of magnetospheric ENA emissions. The ENA imagers observe energetic neutrals produced from global ion populations, over a broad energy range (1–100 keV/u) with high angular (4∘×4∘) and time (about 1-minute) resolution. TWINS distinguishes hydrogen ENAs from oxygen ENAs. Each TWINS spacecraft also carries a Lyman-α geocoronal imager to monitor the cold exospheric hydrogen atoms that produce ENAs from ions via charge exchange. Complementing the imagers are detectors that measure the local charged particle environment around the spacecraft. During its first five years of science operations, TWINS has discovered new global properties of geospace plasmas and neutrals, fostered understanding of causal relationships, confirmed theories and predictions based on in situ data, and yielded key insights needed to improve geospace models. Analysis and modeling of TWINS data have: (1) obtained continuous (main phase through recovery) global ion spectra, (2) revealed a previously unknown local-time dependence of global pitch angle, (3) developed quantitative determination of ion fluxes from low altitude ENAs (4) determined dynamic connections between local pitch angle and global ion precipitation, (5) confirmed local-time dependence of precipitating ion temperature, (6) imaged global dynamic heating of the magnetosphere, (7) explained why the oxygen ring current survives longer into recovery than hydrogen, and (8) revealed new global exospheric density features and their influence upon ring current decay rates. Over the next several years of the solar cycle, TWINS observations of three-dimensional (3D) global ion dynamics, composition, origins and destinies are crucial to capture the system-level view of geospace over the full range of geomagnetic and solar activity conditions.

Radiation Belt Storm Probes: A New Mission for Space Weather Forecasting

In a magnetically insulated diode, collision-free electrons return to the cathode and no electron current is present at the anode. Electron transport to the anode is studied in this paper. Steady-state space-charge-limited flow is assumed initially. Breakdown of ion flow occurs when static neutral atoms at the anode undergo charge exchange, which results in neutral atoms drifting across the diode. These are subsequently ionized by reflexing ions producing electrons trapped in Larmor orbits throughout the diode. These electrons drift to the anode via ionization and inelastic collisions with other neutral atoms. Model calculations compare the effects of foil and mesh cathodes. Steady-state space-charge-limited ion current densities are calculated. The neutral atom density at the cathode is determined as a function of time. The shorting time of the diode is scaled versus the electrode separation d, the diode potential V/sub 0/, the magnetic field, and the initial concentration of static neutron atoms.

We present a highly efficient technique for observing an avoided-crossing signal resulting from modified decay branching ratios of excited atomic states in the vicinity of avoided crossings (ACs) formed in tunable external fields. The technique is based on detection of atoms in metastable (MS) states, and we apply it to the case of photoexcited helium atoms which cascade to the singlet and triplet $1s2s$ states in a dc electric field. We show that narrow structures present in the MS atom yield are due to the increased probability to cascade to the triplet MS states at the field strengths at which the ACs emerge. The resolution of the present technique is not limited by the excitation bandwidth, only by the extent to which a homogeneous field can be produced over a limited region of space. Using results of high-precision calculations which include relativistic and QED corrections, we are able to reproduce the dependence of the measured MS atom yield on the electric-field strength.

Coulomb collisions are important transport processes of charged particles and should not be neglected in atmospheric thermal microplasmas due to the high density of charged particles. However, there have been a few attempts at Coulomb collisions in atmospheric thermal microplasmas, and it is still unclear how to couple Coulomb collisions between different charged particles with collisions between charged particles and neutral atoms. This work proposes a Coulomb collision model based on the collision cross section, which is more computationally efficient than the typical Nanbu model and TA (Takizuka and Abebinary) model by reproducing the anisotropic electron–electron collision process. Then, the effects of Coulomb collision on the kinetic parameters and electron dynamics of atmospheric thermal microplasmas are investigated by coupling the implicit particle in cell and Monte Carlo collision (PIC MCC) method. It is found that the existence of electron–electron and electron–ion collisions reduces the particle number density in the bulk-plasma region. The electron–electron collisions are stronger than the electron–ion collisions and the ion–ion collisions can be ignored. The Coulomb collisions increase the dispersion of electron energy in the high-energy region.

A semiclassical approach is used to calculate excitation and charge transfer cross sections in neutral atom-atom collisions.

Electron capture processes in collisions of O6+ with ground state He and H2 are investigated using the two-centre atomic orbital close-coupling method. Total and state-selective one-electron capture cross sections are obtained for collision energies between 0.5 and 300 keV/u. The comparison with the available experimental state-selective capture data in the overlapping energy range (0.5–100 keV/u for O6++He and 0.5–8 keV/u for O6++H2) shows a good overall agreement. The polarization degrees of 3p 2P3/2–3s 2S1/2 radiation from O5+(3p 2P3/2) produced in O6++He and O6++H2 collisions are calculated from the magnetic substate-selective cross sections with inclusion of cascade contributions from higher n = 4 and n = 5 states. Good agreement is obtained with the experimental data available in the energy range 3–8 keV/u. Below ∼10 keV/u, the polarization degrees of O5+(3p 2P3/2) in both collision systems exhibit an oscillatory structure and above this energy they steadily increase with the increase of collision energy, reaching the values of about 0.37 at 300 keV/u. The energy behaviour of the polarization degree of O5+(3p 2P3/2) in the O6++He collision system is determined almost exclusively by the direct electron capture to 3p0 and 3p1 states of O5+, while in the case of the O6++H2 collision system in the energy region below ∼40 keV/u it is strongly affected by the cascade contributions from the 4l states, which are the dominant capture states in this system.

The arc spectrum of nitrogen has been quite completely excited and measured in the region 8800-3400A with a low voltage arc in a mixture of helium and nitrogen. The process of excitation is probably a two step process: the first being the dissociation of nitrogen molecules into neutral atoms by impacts of the second kind with metastable helium atoms and the second, being the excitation of neutral nitrogen atoms by second impacts with metastable helium atoms. A complete list of wave-lengths is given and the classifications of the lines designated as far as they are known. A considerable number of new lines have been classified.

An overview of experimental data and theoretical computational methods is given for effective cross sections of charge exchange (electron capture) and electron loss (projectile ionization) processes involving heavy many-electron ions (like , , , ) colliding with neutral atoms (H, He, N, Ne, Ar, Kr, Xe) in the energy range, i.e., from low up to relativistic energies. These charge-changing processes can occur with a high probability, reaching cross-section values and, therefore, they play a key role in the kinetics of laboratory and astrophysical plasmas and influence the lifetimes of ion beams in accelerator facilities. Multielectron capture and loss processes are considered, as well, since their importance in the case of heavy atomic projectiles strongly increases, and a contribution to the total cross sections reaches more than 50%. An important aspect of the overview is a consideration of the influence of the inner-shell electrons of two colliding systems and a role of isotope effects in electron capture by very slow ions () from hydrogen isotopes H, D, and T. A short description of the corresponding computer codes is given for the calculation of cross sections of electron capture and electron loss processes for complex atoms and ions over a wide collision energy range.

We use the eikonal approximation to develop a general formula for the cross sections of inelastic collisions of multicharged fast ions (including relativistic ions) with atoms that is applicable within a broad range of collision energies, has the standard nonrelativistic limit, and becomes, in the ultrarelativistic case, the well-known result that follows from the exact solution of the Dirac equation. As an example we study the excitation and ionization of a hydrogenlike atom, the single and double excitation and ionization of a heliumlike atom, and multiple (up to the eighth order) ionization of the neon atom and (up to eighteenth order) ionization of the argon atom. We derive simple analytical expressions for the inelastic cross sections and establish recurrence relations linking the cross sections of ionization of different orders. Finally, we compare our results with the experimental data.