Ionization Processes In Collisions Research Articles

Theoretical analysis of the electronic structure and electron collision ionization process in a strongly coupled semi-classical plasma environment

In this work, we propose a distorted wave method in the relativistic framework to analyze the electron collision ionization process under the influence of strongly coupled semi-classical plasmas. A pseudopotential is used to represent the screened interactions between the core and outer electron, together with solving the Dirac equation to take care of the relativistic effects. This potential characterizes the plasma in terms of the Debye screening length and the de Broglie wavelength, which takes into account both the quantum mechanical effects of diffraction (short distance) and the collective effects (long distance). With this method, the traditional central-field potential is modified, the bound- and continuum-electron wavefunctions and the collision matrix elements are estimated under a relativistic Dirac–Coulomb framework. The strongly coupled semi-classical plasma effects on the energies, transition rates, and the electron impact ionization cross sections are investigated in detail, using the hydrogen atom as an example. Our results agree well with other available theoretical data. The reported results and the computational scheme are not only of interest to the communities such as atomic and plasma physics, but also have implications for many other areas, including astrophysical objects, inertial confinement fusion plasmas, etc.

The Role of Collision Ionization of K-Shell Ions in Nonequilibrium Plasmas Produced by the Action of Super Strong, Ultrashort PW-Class Laser Pulses on Micron-Scale Argon Clusters with Intensity up to 5 × 1021 W/cm2

The generation of highly charged ions in laser plasmas is usually associated with collisional ionization processes that occur in electron–ion collisions. An alternative ionization channel caused by tunnel ionization in an optical field is also capable of effectively producing highly charged ions with ionization potentials of several kiloelectronvolts when the laser intensity q > 1020 W/cm2. It is challenging to clearly distinguish the impacts of the optical field and collisional ionizations on the evolution of the charge state of a nonequilibrium plasma produced by the interaction of high-intensity, ultrashort PW-class laser pulses with dense matter. In the present work, it is shown that the answer to this question can be obtained in some cases by observing the X-ray spectral lines caused by the transition of an electron into the K-shell of highly charged ions. The time-dependent calculations of plasma kinetics show that this is possible, for example, if sufficiently small clusters targets with low-density background gas are irradiated. In the case of Ar plasma, the limit of the cluster radius was estimated to be R0 = 0.1 μm. The calculation results for argon ions were compared with the results of the experiment at the J-KAREN-P laser facility at QST-KPSI.

Generation of rf radiation by low-intensity laser pulse trains in air.

In this paper, we analyze and numerically simulate mechanisms for generating directed rf radiation by a low-intensity laser pulse train (LPT) propagating in air. The LPT ionizes the air, forming a plasma filament. The ionization process relies on the background level of radioactivity which plays an important role in initiating a collisional ionization process. In our model a low-intensity LPT photoionizes background negative ions (produced by ambient ionizing radiation) and provides the seed electrons necessary to initiate collisional ionization. The intensity of the LPT is far below tunneling ionization levels. The ponderomotive forces associated with the LPT and self-fields drive plasma oscillations predominately in the radial direction. The driven radial electron currents in turn generate directed rf radiation. As the plasma density builds up on axis, the later portion of the LPT can defocus and limit the interaction length. The spectrum of the rf radiation consists of the fundamental frequency associated with the pulse separation time as well as harmonics. The rf generation mechanism is analyzed using fluid equations which incorporate, among other things, the effects of background radioactivity, photoionization, collisional ionization, ponderomotive and space-charge effects, and electron attachment-recombination processes. As an example, for a specific set of parameters, the rf spectrum and intensity are compared to experimental data.

Influence of plasma fluctuation on the electronic structures and electron collisional excitation and ionization processes

Studies demonstrated significant discrepancies between the experimental observations of Stillman et al. [Phys. Rev. E 95, 063,204 (2017)] and almost all recent state-of-the art theoretical predictions for the He-α (1s2p→1s2) line shift of Al11+ ions placed in a hot dense plasma environment. Here, we suggest a new distorted wave method to determine the electronic structure and collision dynamics of highly stripped ions placed in plasmas. The method incorporating the plasma fluctuation, which yields modification term to the calculation of the central-field potential, is applied to the above problem. By adjusting the parameter q, which characterizes the intensity of the plasma fluctuation, we reproduce the experimental line shifts well over the whole range (of temperature and density) measured. In addition, we perform a calculation by incorporating the screened potential of Stanton and Murillo [Phys. Rev. E 91, 033,104 (2015)] and the current results (from ab-initio calculations) are in much better agreement with the experimental data than most previous theoretical results. Finally, we perform the calculation of electron collisional excitation and ionisation dynamic processes, where the continuous electron wave function is obtained by numerically solving the modified coupled Dirac equation. A careful investigation of the effect of plasma fluctuation on the integrated cross sections is made. The present work not only leads to further stimulation of the development of shielding models in extreme conditions, but also has important applications in laboratory and space plasmas.

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.

Electron-impact single ionization of the nitrogen atom

We study electron-impact single ionization for the nitrogen atom by performing level-to-level calculations. The present study includes direct and indirect processes for all levels of the ground configuration. We explain experimental data using the scaled distorted wave cross-sections. Cross-sections obtained for the highest level of the ground configuration are ~70% larger than the ground one. The contribution of the indirect process is equal to ~5% for the ground level, but this contribution reaches ~30% for the highest level of the ground configuration. Good agreement for the single ionization cross-sections is obtained with values calculated using the B-spline R-matrix with the pseudostates method. We compare our results with data from the standard R-matrix with pseudostates calculations. The cross-sections and Maxwellian rate coefficients are tabulated for the electron-impact collisional ionization and excitation-autoionization processes.

Electron-impact single, double, and triple ionization of B+

Electron-impact single, double, and triple ionizations are investigated for the ${\mathrm{B}}^{+}$ ion. The direct and indirect ionization processes are studied for the single and double ionization. Good agreement with measurements for the single and double ionization is obtained when the scaled distorted-wave cross sections are used to study the collisional ionization and excitation processes. It is shown that ionization-excitation-autoionization and excitation-ionization-autoionization processes have to be introduced to the total double ionization to explain the measurements. The triple ionization is investigated by considering the direct double ionization with the subsequent autoionization.

Benchmark Experiment to Prove the Role of Projectile Excited States Upon the Ion Stopping in Plasmas.

We report on a precision energy loss measurement and theoretical investigation of 100 keV/u helium ions in a hydrogen-discharge plasma. Collision processes of helium ions with protons, free electrons, and hydrogen atoms are ideally suited for benchmarking plasma stopping-power models. Energy loss results of our experiments are significantly higher than the predictions of traditional effective charge models. We obtained good agreement with our data by solving rate equations, where in addition to the ground state, also excited electronic configurations were considered for the projectile ions. Hence, we demonstrate that excited projectile states, resulting from collisions, leading to capture-, ionization-, and radiative-decay processes, play an important role in the stopping process in plasma.

Monte Carlo Based Glauber Model in Pb-Pb Collisions at LHC

Glauber model is a model used to describe heavy ion collision process. In this paper, we describe an implementation of a Monte Carlo based Glauber Model calculation used for the PHOBOS experiment, which simulates the collision of two heavy nuclei. The nucleon distribution in two initial Pb nuclei is parameterized with Woods-Saxon. The energy profile is √(S_NN )=5.5TeV and inelastic cross-section area, i.eσ_NN= 72mb. Geometric quantities such as impact parameter(b), the participant elliptical and triangular eccentricities (ε₂, ε₃) are studied and presented graphically and quantitatively in this paper. The results for collisions of Pb-Pb are compared with other Glauber model calculations which agrees with the results from other studies to a great extent. In this simulation, a decline trend of ε₂ and ε₃ is observed with increasing Npart. The centralities of peripheral events (40 to 50 percent centrality) are ε₂ ranging from 0.5 to 0.4 and ε₃ being roughly 0.3. The ratio of ε₃ to ε₂ is roughly 0.5 at Npart=0 and 1 at Npart=416 (a hundred percent centrality), showing the significance of triangular flow of the area of intersection. The model can also be used to predict geometric quantities in the LHC experiments and other collision processes.

The mechanism of gas pressure and temperature dependent surface flashover in compressed gas involving gas adsorption

In surface science, when a high voltage is applied, a penetrating discharge occurs along the gas-solid interface, which is called surface flashover. Although the electron stimulated gas desorption is a crucial process for surface flashover in vacuum, the underlying gas-solid interaction physics process for surface flashover is still ambiguous in compressed gas. In this study, the effect of gas adsorption on surface flashover is taken into consideration, surface flashover voltage, gas breakdown voltage, gas adsorption quantity are tested at various gas pressures and temperatures in N2, and the surface gas density is calculated. It is found that the difference between surface flashover voltage and gas breakdown voltage (Vf/Vb) has a positive correlation with the relative gas adsorption ability (Va×T/p or Nsur/Nav) in a wide gas pressure, temperature and density ranges. The results indicate surface flashover occurs in the gas adsorption layer, which density is higher than the average gas density in the gas phase. The gas adsorption improves the surface flashover voltage by impeding electrons collision ionization process in compressed gas. The findings in this research provide a new perspective for studying surface flashover, and offer a route to improve surface flashover performances by tailoring surface adsorption characteristics.

Femtosecond electronic structure response to high intensity XFEL pulses probed by iron X-ray emission spectroscopy

We report the time-resolved femtosecond evolution of the K-shell X-ray emission spectra of iron during high intensity illumination of X-rays in a micron-sized focused hard X-ray free electron laser (XFEL) beam. Detailed pulse length dependent measurements revealed that rapid spectral energy shift and broadening started within the first 10 fs of the X-ray illumination at intensity levels between 1017 and 1018 W cm-2. We attribute these spectral changes to the rapid evolution of high-density photoelectron mediated secondary collisional ionization processes upon the absorption of the incident XFEL radiation. These fast electronic processes, occurring at timescales well within the typical XFEL pulse durations (i.e., tens of fs), set the boundary conditions of the pulse intensity and sample parameters where the widely-accepted ‘probe-before-destroy’ measurement strategy can be adopted for electronic-structure related XFEL experiments.

Electron-impact single ionization of Si+

Electron-impact single ionization is studied in the Si+ ion by considering transitions among energy levels. The study includes excitation-autoionization (EA) and collisional ionization processes. The excitations are investigated up to shells with the principal quantum numbers n ≤ 10. It is shown that correlation effects included in the EA calculations play a crucial role in explaining measurements. The correlation effects diminish the EA cross sections by ∼30% compared to single-configuration calculations. However, the collisional ionization is not significantly affected by an extension of the basis of interacting configurations. Good agreement between the presented level-to-level distorted wave data and experimental measurements is found for the single ionization of the Si+ ion.

Classical simulation of spin-tagged collisional ionization at near-threshold energies

In this paper, we develop a spin dependent classical trajectory Monte Carlo approach to investigate the spin-tagged collisional ionization process. With the help of anti-symmetrized Gaussian wave packets, an effective interaction potential between the two spin-tagged electrons is derived to mimic the spin associated quantum effect of Pauli exclusion principle. Our model is applied to investigate the spin dependent collisional ionization of e–H system. The results are in good agreement with experimental data, ab initio quantum-mechanical calculations as well as a variety of characteristics of the Wannier theory for near-threshold ionization. In contrast, the popular Fermionic molecular dynamics model overestimates the total ionization cross section as high as ten times and predicts a totally inversed ionization spin asymmetry contrary to experimental results. Further applications of our model to laser-assisted collisional ionization are demonstrated and some interesting results are presented.

Angular distribution of characteristic X-ray emission from Fe and V following photoionization

<sec> The de-excitation process of vacancy in the inner shell of the target atom caused by collision ionization produces the characteristic X-ray or Auger electrons. The precise measurement of ionization cross sections plays an important role in many basic research fields, as well as in practical fields, such as chemical analysis of Particle Induced X-ray Emission (PIXE), atomic and nuclear processes, and X-ray fluorescence (XRF) spectroscopy. As we know, when ionization cross sections are measured precisely, whether the emission of X-ray is isotropic in collision process must be considered. However, there have been few experimental results for angular dependence of <i>K</i><sub>β</sub>/<i>K</i><sub>α</sub> intensity ratios in the literature until now. Therefore, this study aims to verify that the <i>K</i><sub>α</sub> and <i>K</i><sub>β</sub> X-rays originated from filling of the <i>K</i> shell vacancies with total angular momentum quantum number 1/2 (<i>J</i> = 1/2) are isotropic.</sec><sec> In this work, the typical <i>K</i>-shell X-ray spectra for Fe and V, which induced by bremsstrahlung with central energy of 13.1 keV, have been measured at emission angles varied from 120° to 170° at intervals of 10°. The characteristic X-ray spectra obtained by the detector are fitted by Gauss function, where the absorption of incident X-rays by the detector, the absorption of emitted X-rays by the atmosphere and the self-absorption correction factor of incident and emitted X-rays by the target are all taken into account.</sec><sec> The experimental results of <i>K</i><sub>β</sub>/<i>K</i><sub>α</sub> intensity ratio in this experiment coincide with those of theoretical calculation, as well as the Ertuğral’s experimental result. The experimental results show that the intensity ratio of <i>K</i><sub>β</sub>/<i>K</i><sub>α</sub> is a constant at different detection angles. Therefore it can be concluded that the emission of <i>K</i><sub>α</sub> and <i>K</i><sub>β</sub> is isotropic in the detection range. Since the <i>K</i> shell has no sub-shell, there is no Coster-Kronig transition in the collision ionization process. In the process of photoionization, the vacancies in the <i>K</i> shell are produced by direct ionization. As a result, the cross section ratio of <i>K</i> shell X-ray generation is independent of the <i>K</i> shell photoionization cross section. In addition, the experimental results show that the <i>K</i><sub>β</sub>/<i>K</i><sub>α</sub> characteristic X-ray intensity ratio of target Fe is 8% higher than that of target V, which are consistent with the theoretical analysis results that the characteristic X-ray intensity ratio depends on the target atomic number <i>Z</i>.</sec>

Transport of ultraintense laser-driven relativistic electrons in dielectric targets

Ultraintense laser-driven relativistic electrons provide a way of heating matter to high energy density states related to many applications. However, the transport of relativistic electrons in solid targets has not been understood well yet, especially in dielectric targets. We present the first detailed two-dimensional particle-in-cell simulations of relativistic electron transport in a silicon target by including the field ionization and collisional ionization processes. An ionization wave is found propagating in the insulator, with a velocity dependent on laser intensity and slower than the relativistic electron velocity. Widely spread electric fields in front of the sheath fields are observed due to the collective effect of free electrons and ions. The electric fields are much weaker than the threshold electric field of field ionization. Two-stream instability behind the ionization front arises for the cases with laser intensity greater than $5\times 10^{19}~\text{W}/\text{cm}^{2}$ that produce high relativistic electron current densities.

Electron-impact ionization of Fe3+

SynopsisElectron-impact ionization cross sections are studied for levels of the ground and first excited con-figurations of the Fe3+ ion. Convergence for the excitation-autoionization cross sections due to excitations to the high-nl shells is investigated. Single configuration and configuration interaction methods are used in the study. Scaling is applied to the distorted wave cross sections of the excitation-autoionization and collisional ionization processes.

Simulation Study on Positive Corona Discharge of Receptors on Rotating Wind Turbine Blade Tips under Thundercloud Electric Fields

Recent lightning simulation experiments have not simulated the influence of blade rotation in the long term after corona inception. This study uses a finite element method and considers the adhesion and collision processes of positive ions, aerosol ions, and neutral particles to establish a two-dimensional positive corona discharge model based on a multicomponent diffusion transport equation. The microscopic distribution of these three types of particles and the influence mechanism of charged particles’ migration under electric fields and wind were studied. The results show that ion migration is affected by both electric field and wind speed. The higher the wind speed, the larger is the deviation amplitude of charged particles along the direction of wind. With an increase in wind speed, the corona current on the receptor surface gradually increases. When the wind speed is 30 m/s, the corona current peak value increases by almost six times when compared with that when no wind is present. From this, it can be inferred that blade rotation reduces the concentration of positive ion in the receptor area, thereby facilitating electron avalanche and streamer-leader conversion.

The role of multiple ionization of H2O in heavy ion collisions

Collisional ionization processes involving H2O molecules and C6+, O8+, Si13+ ions are studied by means of the classical trajectory Monte Carlo method using molecular orbital calculations to define the ionization stages of the water molecule. Net total and single-differential cross sections in energy and angle are obtained by using a newly developed model that goes beyond the commonly applied one-active electron approximation. This model allows us to access the fraction of electron emission arising from single and multiple electron ionization. Calculated cross sections are contrasted and benchmarked against available experimental data at impact energies in the MeV/u range. The present results highlight the important role of multiple ionization in the emission of electrons where we find the majority of electrons emitted with energies greater than ~50 eV arise from multiple ionization collisions.

Atom-Rydberg Atom Collisions in Hydrogen Plasmas: Cross Sections and Rate Coefficients

Non-LTE modelling requires accurate atomic data e.g. collisional excitation and ionization cross-sections and rate coefficients. In order to improve the modeling of the solar photosphere, as well as to model atmospheres of other similar and cooler stars where the main constituent is also hydrogen, it is necessary to take into account the influence of all the relevant collisional processes on the excited-atom populations in weakly ionized hydrogen plasmas. In this context we present the data needed for the inclusion of the specific atomic collisional processes in the investigation of the optical and kinetic properties of weakly ionized stellar atmospheres layers. The ionization processes in collisions of excited hydrogen atoms with atoms in ground states were considered for the principal quantum numbers 2 ≤ n ≤ 20 and temperatures 4000 K ≤ T ≤ 20000 K.

Hydrodynamical interaction of stellar and planetary winds: effects of charge exchange and radiation pressure on the observed Ly α absorption

ABSTRACT Lyman α observations of the transiting exoplanet HD 209458b enable the study of exoplanet exospheres exposed to stellar extreme ultraviolet (EUV) fluxes, as well as the interacting stellar wind properties. In this study we present 3D hydrodynamical models for the stellar–planetary wind interaction including radiation pressure and charge exchange, together with photoionization, recombination, and collisional ionization processes. Our models explore the contribution of the radiation pressure and charge exchange to the Ly α absorption profile in a hydrodynamical framework, and for a single set of stellar wind parameters appropriate for HD 209458. We find that most of the absorption is produced by the material from the planet, with a secondary contribution of neutralized stellar ions by charge exchange. At the same time, the hydrodynamic shock heats up the planetary material, resulting in a broad thermal profile. Meanwhile, the radiation pressure yields a small velocity shift of the absorbing material. While neither charge exchange nor radiation pressure provides enough neutrals at the velocity needed to explain the observations at −100 km s−1 individually, we find that the two effects combined with the broad thermal profile are able to explain the observations.