Charge transfer and excitation in high-energy ion-atom collisions

Coincidence measurements of charge transfer and simultaneous projectile electron excitation provide insight into correlated two-electron processes in energetic ion-atom collisions. Projectile excitation and electron capture can occur simultaneously in a collision of a highly charged ion with a target atom; this process is called resonant transfer and excitation (RTE). The intermediate excited state which is thus formed can subsequently decay by photon emission or by Auger-electron emission. Results are shown for RTE in both the K-shell of Ca ions and the L-shell of Nb ions, for simultaneous projectile electron loss and excitation, and for the effect of RTE on electron capture.     

Dynamics of molecular collisions with surfaces: Excitation,dissociation and diffraction

Aspects of molecular collisions with surfaces are discussed which are important in the chemically relevant energy range of ∼ 1–10 eV. In particular, the role of charge transfer, potential energy surface topology and intramolecular ground and excited-state potential curves are investigated as they pertain to internal vibrational excitation, dissociative adsorption or scattering, and diffractive scattering. The modeling and analysis is based on classical trajectories and semiclassical wavepacket dynamics, both for intramolecular and translation     

Recent Developments in Accelerator Based Atomic Physics

Much progress toward a detailed understanding of atomic collision phenomena and atomic structure has recently been made with the aid of accelerated ion beams. Beam foil Lamb shift measurements and the possible observation of spontaneously created positrons from quasi-superheavy atoms give promise for critical tests of QED. The mechanisms of innershell ionization and charge transfer in ion atom collisions have been intensively investigated and the results have brought together the various approaches - Coulomb excitation and MO treatments for these processes. That violent collisions at high energies can cause multiple ionization of the target atom has been known for some time. However, only recently have theoretical treatments been able to predict, e.g., the degree of L shell ionization accompanying K shell ionization. The highly charged ions formed in these collisions have themselves come under intensive investigation and are used for the study of the structure and collision physics of multiply charged species. Electron-Ion collision studies have also been started and initial results on dielectronic recombination are reported.     

Ionization and excitation collision processes of electrons in liquid water

This paper reports on Monte Carlo simulations of electrons in liquid water using a set of electron collision cross-sections constructed with data published recently. The track history of electrons having initial energy ranged from 1 keV to 10 keV is investigated looking at the ionization and excitation processes. The results show that the ratio of the ionization and excitation events per track history is unique independent of the initial electron energy above a couple of 100 eV and these inelastic processes occur with low energy electrons frequently below 100 eV. In particular, the excitation processes are dominated by the electrons below 50 eV. Flight distance distributions between the inelastic collisions are also discussed.     

Non-equilibrium kinetic versus Monte Carlo modelling of hydrogen-surface interactions

The study presents a contribution to the modelling of processes with H₂ molecules in both ground and vibrationally excited state and with H atoms. Attention is focused on the surface processes, which besides volume collisions, can influence significantly the vibrational population of hydrogen molecules. It is important to know this population since it opens a variety of low-energy channels at the cold edge of fusion plasmas. We have developed a simple kinetic model to estimate the particle fluxes formed by vibrational excitation and dissociation of hydrogen on metal surfaces, at low pressures and low temperatures. Comparison with the experiment shows that it grasps the main of the underlying physics: further improvements accompanied by more complete vibrational state-selective data are foreseen.     

A One and a Half Centered Expansion for Ion Atom Collisions

A new method for numerically calculating ion atom collisions is presented. It uses time dependent amplitudes for target excitation and ionization but time independent amplitudes for charge transfer. It is efficient in its use of computer time and removes the known difficulty with a single centered expansion for the proton hydrogen system below 60 keV.     

Database for inelastic collisions of sodium atoms with electrons, protons, and multiply charged ions

The available experimental and theoretical cross section data for inelastic collision processes of ground (3s) and excited (3p, 4s, 3d, 4p, 5s, 4d, and 4f) state Na atoms with electrons, protons, and multiply charged ions have been collected and critically assessed. In addition to existing data, electron-impact cross sections, for both excitation and ionization, have been calculated using the convergent close-coupling approach. In the case of proton-impact cross section, the database was enlarged by new atomic-orbital close-coupling calculations. Both electron-impact and proton-impact processes include excitation from the ground state and between excited states (n = 3-5). For electron-impact, ionization from all states is also considered. In the case of proton-impact electron loss, cross sections (the sum of ionization and single-electron charge transfer) are given. Well-established analytical formulae used to fit cross sections, published by Wutte et al. and Schweinzer et al. for collisions with lithium atoms, were adapted to sodium. The “recommended cross sections” for the processes considered have been critically evaluated and fitted using the adapted analytical formulae. For each inelastic process the fit parameters determined are tabulated. We also present the assessed data in graphical form. The criteria for comprehensively evaluating the accuracy of the experimental data, theoretical calculations, and procedures used in determining the recommended cross sections are discussed.     

Simultaneous Electron Capture and Excitation in Ion-Atom Collisions

A review of recent efforts to observe simultaneous electron capture-and-K-shell excitation in ion-atom collisions is presented. This process which has been referred to as resonant-transfer-and-excitation (RTE), is qualitatively analogous to dielectronic recombination (inverse Auger transition) in free-electron-ion collisions, and, hence, is expected to be resonant. Experimentally, events having the correct signature for simultaneous capture-and-excitation are isolated by detecting projectile K x rays in coincidence with ions which capture a single electron. In a recent experiment involving 70-160 MeV S13+ ions incident on Ar, a maximum was observed in the yield of projectile K x rays associated with electron capture. This maximum is attributed to simultaneous capture-and excitation. The position (120 MeV) and width (60 MeV) of the observed maximum are in good agreement with theoretical calculations. The data indicate that RTE is an important mechanism for inner-shell vacancy production in the energy range studied.     

Ab initio Calculations of Electron-Impact Excitation Cross Sections for N_2 Molecule

One of the great difficulties in understanding nitrogen plasma elementary processes is the lack of an available database of the cross-sections of electron-impact excitations and radiations. Ab initio calculations of vibrational excitation cross sections for electron collisions with nitrogen molecules in low-lying states using similarity function approach, such as a-a＇, a-w, B-B＇ and B-W transition systems, are reported here for the first time. In the meantime, the average excitation energies of neighboring levels of these systems have been calculated. In order to obtain the cross sections, accurate spectroscopic constants and transition dipole moments have been investigated. Potential energy curves and other electronic transition dipole moments for the low-lying states of N2 have been re-evaluated using complete active space self-consistent field （CASSCF） approach with aug-cc-pVqZ basis set. The calculated cross-sections could provide a database for studying the elementary processes and the properties in N2 plasma.     

传热管与支撑件随机碰撞实验研究

针对蒸汽发生器中传热管与支撑件的碰撞行为,对悬臂梁固定的传热管在不同支撑条件下开展了激振实验,获得了传热管均方根位移与接触率,分析了传热管与支撑件磨损功率的变化规律,并探究了传热管固有频率对振动特性的影响。结果表明,防振条支撑与波纹带支撑时传热管的法向均方根位移均随激振力增加逐渐放缓,而防振条支撑对应的切向位移呈线性增长。防振条支撑与波纹带支撑时的接触率均表现为随激振力增大趋于稳定,其中间隙对防振条支撑的接触率影响更明显。在以冲击为主导的激励方式下,激振力与磨损功率表现为明显的正相关。支撑间隙对磨损功率的影响相对复杂,防振条支撑下磨损功率在0.1 mm和0.25 mm间隙存在极值,而波纹带支撑磨损功率仅在0.2 mm间隙存在极值。传热管固有频率对振动响应结果的影响很小。     

Charge transfer processes in astrophysical plasmas

Charge transfer processes involving multiply-charged ions modify the ionization structure of astrophysical plasmas and exert a major influence on plasmas created by the absorption of high frequency radiation. As a recombination mechanism, charge transfer suppresses the higher ionization stages. In hot plasmas, charge transfer ionization may enhance the abundances of highly charged systems. Charge transfer excitation also occurs and the resulting emission lines provide a unique diagnostic probe of the neutral content of the plasma, the ionization distribution and the nature of the ionization source.Examples of the role of charge transfer as a recombination, ionization and excitation mechanism in astrophysical plasmas are presented.     

An L2 representation of the continuum in collisions between alpha particles and lithium atoms

We report new theoretical results obtained in close-coupling calculations within the impact parameter method, using two-centred AO expansions with an extended L² basis for charge transfer, target excitation, and ionisation in collisions between H²⁺ and Li. The basis is chosen by demanding a good representation of oscillator strengths for dipole transitions both to excited and to ionised levels. The calculations bring the theoretical and experimental total cross sections for single-electron capture between 40 and 200 keV lab. close together. They also reproduce accurately the Li(2p) excitation cross sections over a wide energy range. The first close-coupling estimate of ion-impact ionisation of Li is also given. Preliminary results suggest that an important contribution to the ionisation cross sections at intermediate energies may come from transitions to d- and f-states of the target continuum.     

Shell effect on stabilization processes following multi-electron transfer in slow Arq+–Ar (q = 6–9, 11) collisions

We experimentally investigate the shell effect on the stabilization processes following the multi-electron transfer in slow collisions of Ar^q+–Ar (q = 6–9, 11). The relative cross-section ratios of multi-electron transfer and of the subsequent stabilization with respect to single-electron capture are measured meanwhile compared with the theoretical results predicted by the classical over-barrier model. Our result indicates that the multi-electron transfer is dominant when the projectile charge is large and the subsequent stabilization shows a dramatic variation if the projectile L-shell configuration becomes open. It shows that the subsequent stabilization processes of multiply excited scattering ions have a strong dependence on the projectile shell.     

The relaxation of strong electron excitations in solids induced by multicharged ion bombardment

The relaxation of electron excitations arising from irradiation by multicharged ions of metals and insulators is considered. The excitation processes in solids are described. In metals, the relaxation is determined by the electron thermal conductivity, and electron energy transfer to the lattice is small. In insulators, the electron excitation relaxation is dependent on the low temperature ionization wave, the electron heat is localized, and lattice atoms receive enough energy for melting. The electron-phonon interaction at high electron temperatures and lattice relaxation is considered.     

The Role of Metastable Atoms in a Homogeneous Atmospheric Pressure Barrier Discharge in Helium

A one-dimensional fluid model for homogeneous atmospheric pressure barrier discharges in helium is presented by considering elementary processes of excitation and ionization including a metastable atom effect. Using this model we investigate the behaviours of the helium metastable atoms in discharges as well as their influence on the discharge characteristics. It is shown that the metastable atoms with a relatively high concentration during the discharge are mainly produced in the active phase of the discharge and dissolved in the off phase. It is also found that the metastable atom collisions can not only provide seed electrons for discharges but also influence the concentration of ions. A reduction of matestable atom density results in a drop in the charged particle densities and causes a qualitative change in the discharge patterns.     

Collision cascades induced by aluminum clusters impacting on gold thin films

The collision cascades of 0.2keV/atom aluminum clusters impacting on gold thin films were investigated. The energy spectra and the number density as a function of time were calculated by molecular dynamics simulation. The results were compared with monatomic bombardments for the same energy per atom. Aluminum atoms with energies larger than their initial energy were found. The highest energy of gold recoils is about 5 times that allowed in an isolated two-body collision. Craters were found on the target surfaces. Multiple successive collisions and collisions between moving atoms are considered to be important in producing such high energy atoms. The dependence of the energy transfer on the mass ratio and the scattering geometry of two colliding atoms are also discussed.     

Optical excitations in electron-induced desorption of sodium from NaCl surface

Optical excitation processes stimulated by electron bombardment of alkali halides have been investigated under various experimental conditions. As an example a 500 eV e⁻ beam was used to bombard a (100) NaCl crystal. The work included measurements of both excited and ground state sodium atom yields as a function of the target temperature and the beam intensity, a cross-beam experiment with two electron beams: parallel and perpendicular to the sample surface, and a modulated beam measurement in search for time correlations between primary electrons and excited state production.The results were compared with the H-centre migration model and the results of recent publications. In particular it was found that a direct excitation of sodium atoms took place above the surface in collisions with the flux of primary and secondary electrons.     

利用加速器产生的中低能离子束进行离子原子碰撞研究

通过位置灵敏技术和飞行时间法,利用加速器产生的离子束研究中低能低电荷态Xq++B(X=C,O;q=1,2;B=Ne,Ar)的碰撞反应,分析研究了碰撞过程中的转移电离与单电子俘获。对于确定的电荷态,给出了转移电离与单电子俘获截面比值R随入射离子能量变化的规律与单电子电离随能量的变化规律之间的相互关系,同时通过实验数据分析、理论计算及模型对比给出R的峰位的近似表达式。     

Environmental Influences on K X-Rays from Highly Stripped Sulfur Ions Teavelng in Solids

As a fast heavy-ion passes through a material medium, it undergoes multiple collisions in which electrons are continuously lost and recaptured by the ion, and as a result, an equilibrium distribution of excited states is quickly established. The high density of electrons in a solid state environment enables these collisional excitation and decay processes to compete with x-ray and Auger decay. As a result, the widths of the x-ray lines exhibit a dependence on electron density due to collisional broadening. In addition, the potential of the ion is modified by the polarization of loosely bound electrons of the medium. In effect, these electrons act to screen the ion potential, thereby causing the core electron binding energies of the ion to decrease. As a result of this "dynamic screening" effect, the x-ray transition energies inside the medium differ from those in vacuum.     

Numerical study of low-frequency discharge oscillations in a 5kW Hall thruster

A two-dimensional particle-in-cell plasma model is built in the R–Z plane to investigate the lowfrequency plasma oscillations in the discharge channel of a 5 kW LHT-140 Hall thruster. In addition to the elastic, excitation, and ionization collisions between neutral atoms and electrons,the Coulomb collisions between electrons and electrons and between electrons and ions are analyzed. The sheath characteristic distortion is also corrected. Simulation results indicate the capability of the built model to reproduce the low-frequency oscillation with high accuracy. The oscillations of the discharge current and ion density produced by the model are consistent with the existing conclusions. The model predicts a frequency that is consistent with that calculated by the zero-dimensional theoretical model.