Electron-impact Excitation Processes Research Articles

Excitation processes in experimental photoionized plasmas

Photoionized plasmas widely exist nearby strong radiative sources in the universe. With the development of the high energy density facilities, photoionized plasmas related to astrophysical objects are generated in laboratories accordingly. RCF (radiative collisional code based on the flexible atomic code) is a theoretical model applied to steady-state photoionized plasmas. Its rate equation includes five groups of mutually inverse atomic processes, which are spontaneous decay and photoexcitation, electron impact excitation and deexcitation, photoionization and radiative recombination, electron impact ionization and three body recombination, autoionization and dielectronic capture. All of the atomic data are calculated by FAC (the flexible atomic code), and with four input parameters, RCF can calculate the charge distribution and emission spectrum of the plasma. RCF has well simulated the charge state distribution of a photoionizing Fe experiment on Z-facility and the measured spectrum of photoionizing Si experiment on GEKKO-XII laser facility. According to the simulation results, the importance of photoexcitation and electron impact excitation processes in the two photoionization experiments is discussed. In the photoionizing Fe experiment condition, high energy photons not only ionize the ions by photoionization directly, but also excite the ions to autoionizing levels, ionizing the ions indirectly. What is more, far from ionizing the ions, electrons even suppress the ionization of the plasma by exciting the ions to levels with small ionization cross sections. In the photoionizing Si experiment condition, because of high photoexcitation rate, strong resonance line of He-like ion and some Li-like ion lines, which have similar spontaneous decay rates as the resonance line, are emitted. Although the intercombination line of He-like ion has lower spontaneous decay rate than the resonance lines, strong recombination makes them have comparable strengthes. Electron impact excitation can influence the line ratio of He-like ion lines by affecting the distribution of 1s2l (l=s,p) levels.

Unified calculation of generalized oscillator strength of argon ranging from bound to continuum states

The electron and photon scattering data of an atom are crucial for many scientific fields, including plasma physics, astrophysics, and so on. For high enough but nonrelativistic incident energies, the first Born approximation is applicable for calculating these data, in which the key physics quantity is the generalized oscillator strength (GOS). In high-energy electron impact excitation processes, atoms will be excited into various excited states including strongly perturbed Rydberg and adjacent continuum states. How to calculate these quantities of a nontrivial many-electron atom rapidly and accurately is still a great challenge. Based on our eigenchannel $R$-matrix method $R$-eigen, we further extend it to calculate the GOS of a whole channel in an atom, which includes all Rydberg and adjacent continuum states. The ${J}^{\ensuremath{\pi}}={1}^{\ensuremath{-}}$ states of argon are chosen as an illustrating example. The calculation results are in good agreement with the available benchmark absolute experimental measurements. The calculated eigenchannel GOS matrix elements are smooth functions of the excitation energy and momentum transfer. From such smooth eigenchannel GOS matrix elements, we can obtain the GOS of any specific excited state through multichannel quantum defect theory, e.g., infinite Rydberg (including a strongly perturbed one), autoionization, and continuum states.

Circular polarization of the Lyman-α 1 line radiation emitted by longitudinally-polarized electron impact excitation: effects of Breit interaction and radiation multipoles

Detailed calculations are carried out for the longitudinally-polarized electron impact excitation cross sections from the ground state 1s 1/2 to the individual magnetic sublevels of the excited state 2p 3/2 of highly charged H-like ions using a fully relativistic distorted-wave method. The influence of the Breit interaction and radiation multipoles on the circular polarization of X-ray radiation are investigated systematically. Our results show that the Breit interaction, as the dominant correction to the Coulomb interaction, may lead to a considerable change in the magnetic sublevels cross sections and circular polarizations of Lyman-α 1 (2p 3/2 → 1s 1/2) line radiation with a more pronounced effect at higher incident energies and/or atomic numbers. Moreover, the interference between the electric dipole (E1) decay and the magnetic quadrupole (M2) decay may significantly alter the circular polarizations of the emitted X-rays. The circular polarizations of the Lyman-α 1 line are exhibit a different behavior than the linear polarizations for the same transition line regarding the radiative electron capture [Weber et al., Phys. Rev. Lett. 105, 243002 (2010)], radiative recombination [Bettadj et al., J. Phys. B 47, 105205 (2014)], and electron-impact excitation processes [Chen et al., Phys. Rev. A. 90, 012703 (2014)].

Ground-state excitation of heavy highly-charged ions

We have studied the excitation of H-like and He-like uranium (U91+ and U90+) in relativistic collisions with gaseous targets by observing the subsequent x-ray emission. The experiment was conducted at the ESR storage ring of the GSI accelerator facility in Darmstadt, Germany. The measurements were performed with a newly developed multi-phase target at different collision energies. This enabled us to explore the proton (nucleus) impact excitation as well as the electron impact excitation processes in the relativistic collisions. The large fine-structure splitting in uranium allowed us to unambiguously resolve excitation to different L-shell levels. Moreover, information about the population of different magnetic sublevels has been obtained via an angular differential study of the decay photons associated with the subsequent de-excitation process. The experimental results are compared with calculations performed within the relativistic framework including excitation mechanisms due to both protons (nucleus) and electrons.

The contribution of the magnetic quadrupole to the angular distribution and polarization of electron-impact excitation and dielectronic recombination in highly charged ions

The degree of linear polarization and angular distribution of the x-ray photoemission of highly charged He-like and Li-like uranium ions following electron-impact excitation and dielectronic recombination processes are calculated using a fully relativistic distorted-wave method. A detailed investigation is carried out regarding the contribution of the magnetic quadropole (M2) term to the subsequent characteristic x-ray emission from the above two different processes. It is found that while the M2 term has a slight effect on the angular distribution and linear polarization of electron-impact excitation, it has a substantial effect on the same properties of dielectronic recombination.

The role of electron-impact vibrational excitation in electron transport through gaseous tetrahydrofuran.

In this paper, we report newly derived integral cross sections (ICSs) for electron impact vibrational excitation of tetrahydrofuran (THF) at intermediate impact energies. These cross sections extend the currently available data from 20 to 50 eV. Further, they indicate that the previously recommended THF ICS set [Garland et al., Phys. Rev. A 88, 062712 (2013)] underestimated the strength of the electron-impact vibrational excitation processes. Thus, that recommended vibrational cross section set is revised to address those deficiencies. Electron swarm transport properties were calculated with the amended vibrational cross section set, to quantify the role of electron-driven vibrational excitation in describing the macroscopic swarm phenomena. Here, significant differences of up to 17% in the transport coefficients were observed between the calculations performed using the original and revised cross section sets for vibrational excitation.

Prediction of Nonequilibrium Air Plasma Radiation Behind a Shock Wave

The absolute radiation measurements obtained in the electric arc-driven shock-tube facility at NASA Ames Research Center were analyzed to test the collisional-radiative model developed at École Centrale Paris. Two conditions representative of Earth reentry at 10.54 and were investigated in the vacuum ultraviolet and infrared spectral ranges. For each of the conditions, the corresponding charge-coupled device images were analyzed. The electron number density was inferred from Stark-broadened nitrogen and lines. Comparisons with the predicted electron number density profiles enabled us to validate the ionization rate constant model implemented in the flowfield solver and to accurately locate the shock front. For both freestream conditions and all the spectral ranges, the predictions of the initial intensity rises were improved when the total spatial smearing (due to the shock motion, the optics, and the camera) was taken into account. The nonequilibrium intensities observed in the vacuum ultraviolet and infrared spectral ranges were underpredicted by the collisional-radiative model when only electron-impact excitation and ionization processes were taken into account. Then, the effect of heavy-particle impact processes was studied by applying various multitemperature dissociation rate constant and vibration–dissociation coupling models as well as heavy-particle impact excitation models. The nonequilibrium peak intensities observed in the vacuum ultraviolet and infrared spectral ranges were shown to be controlled by heavy-particle impact excitation processes.

Nanosecond plasma enhanced H2/O2/N2 premixed flat flames

The effect of nanosecond pulsed plasma discharges on a laminar, lean (ϕ=0.5) premixed H2/O2/N2 flame is studied at low pressure (25Torr) using a novel plasma–flame facility, non-intrusive laser diagnostics, and high-fidelity numerical simulations. Spatially-resolved, quantitative OH mole fraction and temperature measurements are performed with and without a burst of 200 discharge pulses using laser-induced fluorescence. Measured temperatures increase by ∼20% in both the pre-heat and post-flame zones with the use of the plasma discharge. In addition, OH mole fractions increase by as much as 500% in the preheat zone and by 40% in the post-flame gases. Simulations are conducted with a one-dimensional, multi-scale, pulsed-discharge model with detailed plasma-combustion kinetics to develop additional insight into the complex plasma and flame interactions. Good agreement between measured and predicted OH and temperature profiles provides confidence in the model framework. The reduced electric field, E/N, varies inversely with number density during each pulse. A significant fraction of the input energy is consumed during electron impact ionization processes in the high E/N (700–1000Td) regions downstream of the flame. Lower E/N values (100–700Td) in the lower-temperature preheat zone promote efficient generation of radicals and excited species via electron impact processes, as well as by collisional quenching of excited states. The plasma action results in a significant increase in O and H atom density, with the peak values increasing by a factor of 6 and 4 respectively. In presence of the discharge, species and temperature profiles shift upstream (closer to the burner) by approximately 0.2cm. Simulations show that electron impact dissociation and excitation processes in the plasma have a major impact on the observed temperature and species profile displacement, such that Joule heating alone does not account for the observed effects of the plasma.

Electron-impact resonant vibrational excitation and dissociation processes involving vibrationally excited N2 molecules

Resonant vibrational excitation cross sections and the corresponding rate coefficients for electron–N2 collisions occurring through the resonant state are reviewed. New calculations are performed using accurate potential energy curves for the N2 electronic ground state, taken from the literature, and for the resonant state, obtained from R-matrix calculations. The calculations are extended to resonant excitation processes involving the N2 ground state vibrational continuum, leading to dissociation. Electron-impact dissociation is found to be significant from higher vibrational levels. Accurate analytical fits for the complete set of the rate coefficients are provided. The behavior of the dissociative cross sections is investigated for rotationally excited N2 molecules, with J = 50, 100 and 150, and for different vibrational levels.

Electron-impact excitation and single- and multiple-ionization cross sections of heavy ions: Sn13+as an example

The cross sections of electron impact with Sn${}^{13+}$ resulting in the production of Sn${}^{13+}$-Sn${}^{16+}$ are investigated theoretically by using the fine-structure-level distorted-wave approximation from the threshold to 4000 eV. The electron-impact excitation, ionization, and resonant excitation processes are included in the cross sections. Contributions from different processes are determined by independent processes and isolated resonance approximations. The decay pathways and branching ratios for single and sequentially multiple autoionization for the final autoionized states are determined by detailed calculations. The cross section forming Sn${}^{13+}$ is the largest and next are those for Sn${}^{14+}$ and Sn${}^{15+}$. The double-ionization cross section forming Sn${}^{15+}$ is smaller than that forming Sn${}^{14+}$, yet the relative magnitude is larger at higher incident electron energy. A comparison with a recent measurement for the single-ionization cross section [A. Borovik, Jr. et al., J. Phys. B 46, 175201 (2013)] showed that accurate determination of the branching ratios for single and double autoionization and including contributions of the resonant excitation double autoionization are crucial to obtain accurate results.

Polarization of the nf → 3d (n=4, 5, 6) x-rays from tungsten ions following electron-impact excitation and dielectronic recombination processes

Electron-impact excitation and resonant electron capture cross sections to the specific magnetic sublevels as well as the polarization of the strong nf→3d (n=4, 5, 6) x-ray emitted from Ni-like through Ge-like tungsten ions have been studied systematically by using a fully relativistic distorted-wave method. We compare the polarizations for the same x-ray but from the above two different processes for the first time. It is found that the polarization degrees of x-ray following both the electron-impact excitation and the dielectronic recombination are totally different. Because the electron-impact excitation dominates in hot and dense plasmas and the dielectronic recombination plays a key role in low-density plasmas which are cooled and recombining by capturing continuum electrons, we hope that the specific plasmas conditions could be assessed by using these x-ray polarizations.

Electron- and Proton-Impact Excitation of Hydrogenlike Uranium in Relativistic Collisions

The K shell excitation of H-like uranium (U(91+)) in relativistic collisions with different gaseous targets has been studied at the experimental storage ring at GSI Darmstadt. By performing measurements with different targets as well as with different collision energies, we were able to observe for the first time the effect of electron-impact excitation (EIE) process in the heaviest hydrogenlike ion. The large fine-structure splitting in H-like uranium allowed us to unambiguously resolve excitation into different L shell levels. State-of-the-art calculations performed within the relativistic framework which include excitation mechanisms due to both protons (nucleus) and electrons are in good agreement with the experimental findings. Moreover, our experimental data clearly demonstrate the importance of including the generalized Breit interaction in the treatment of the EIE process.

Degrees of polarization of the two strongest5f→3dlines following electron-impact excitation and dielectronic recombination processes of Cu-like to Se-like gold ions

Cross sections for electron-impact excitation and capture to individual magnetic sublevels of Cu-like to Se-like gold ions have been calculated by using a fully relativistic distorted-wave (RDW) method. These magnetic cross sections have further been used to obtain the degrees of linear polarization for the first two strongest $5f\ensuremath{\rightarrow}3d$ radiative lines. A detailed comparison has been made between polarizations of these lines from the electron-impact excitation process and those of the same lines but from the dielectronic recombination process. It is found that for a Cu-like gold ion the polarization in the electron-impact excitation process increases sharply before starting to decrease at higher incident electron energies, and it reaches the maximum at around twice the threshold energy. The situations for other gold ions are very similar. However, as for the dielectronic recombination process, the polarization changes among a very large range with different gold ions. Therefore, the obvious differences between the polarizations from different formation processes can be employed in diagnosing the formation mechanism of the corresponding lines.

Theoretical study on electron-impact excitation processes and the relevant polarization of radiation of 2s1/2—2p3/2 in W65+ through W71+

In this paper, the excitation energy and radiative transition probabilities are calculated for 2p3/2-2s1/2 transition in W65+ through W71+ ions, by using GRASP92 package based on the multi-configuration Dirac-Fock method. The present calculations are compared with other theoretical and experimental results [Podpaly et al. 2009 Phys. Rev. A 80 052504], and they are in good agreement each other. Furthermore, the total cross section and the magnetic sublevels cross section for the 2s1/2-2p3/2 excitation in W65+ through W71+ ions as well as the polarization of resulting transitional lines are calculated, with a fully relativistic distorted-wave method. Based on the calculation, the variations of the excitation cross sections and polarization with the increase of incident electron energy are discussed systematically.

Polarization and anisotropic emission of K-shell radiation from heavy few electron ions1This article is part of a Special Issue on the 10th International Colloquium on Atomic Spectra and Oscillator Strengths for Astrophysical and Laboratory Plasmas.

The population of magnetic sublevels in hydrogen-like uranium ions has been investigated in relativistic ion–atom collisions by observing the subsequent X-ray emission. Using the gas target at the experimental storage ring facility we observed the angular emission of Lyman-α radiation from hydrogen-like uranium ions. The alignment parameter for three different interaction energies was measured and found to agree well with theory. In addition, the use of different gas targets allowed for the electron-impact excitation process to be observed.

Calibration of distorted wave Born approximation for electron impact excitation of Ne and Ar at incident energies below 100 eV

We calibrate the distorted wave Born approximation (DWBA) for electron impact excitation processes empirically. Differential cross sections (DCSs) for the excitation of the 2p53s, 2p53p, 2p54s and 2p54p configurations of Ne and the 3p54s and 3p54p configurations of Ar by electron impact are calculated using DWBA for incident energies between 20 and 100 eV. The calculated results are compared with the absolute experimental measurements and other theoretical results. We found that the structure of the DCS can be well reproduced by the DWBA model while the magnitude is overestimated for most cases considered here. The differences in magnitude between DWBA and experiment are used to test the calibration of DWBA such that the DWBA can be used to describe laser-induced electron impact excitation processes. These processes are involved in the non-sequential double ionization of atoms in strong laser fields.

The influence of electron collisions on non-LTE Li line formation in stellar atmospheres

Chemical abundances as measured from stellar spectral lines are often subject to uncertainties due to lack of accurate data for inelastic collisions, which is needed for non-local thermodynamic equilibrium (non-LTE) modelling. For cool stars, understanding of collision processes with electrons and hydrogen atoms is required to achieve high precision measurements. In this thesis, I have investigated the role of these collisions on the non-LTE formation of Li and Mg spectral lines in late-type stars.In the case of Li, electron impact excitation processes were calculated using the R-matrix with pseudo states method and the results found to agree well with recent calculations using the convergent close-coupling technique. These modern data were employed in non-LTE calculations by updating an existing model atom, which already included modern data for hydrogen collision processes. Our results were compared with calculations using older semi-empirical approximation calculations and only small differences were found: about 0.01 dex (~ 2%) or less in the abundance corrections. We therefore conclude that the influence of uncertainties in the electron collision data on non-LTE calculations is negligible. Indeed, together with the collision data for the charge transfer process Li + H ↔ Li+ + H- now available, and barring the existence of an unknown important collisional process, the collisional data in general is not a source of significant uncertainty in non-LTE Li line formation calculations.In the case of Mg, electron impact excitation processes were again calculated with the Rmatrix with pseudo states method, and used together with recent hydrogen collision calculations to build and test a model atom, without free parameters, for non-LTE modelling. Both electron and hydrogen collision processes, including charge transfer and excitation, are found to be important thermalising agents in various cases. The modelled spectra agree well with observed spectra from benchmark stars in the optical and infrared. The modelling predicts non-LTE abundance corrections ∆A(Mg)NLTE–LTE in dwarfs, both solar metallicity and metal-poor, to be very small (of order 0.01 dex), even smaller than found in previous studies. In giants, corrections vary greatly between lines, but can be as large as 0.4 dex. Results of calculations in a large grid of 1D model atmospheres are presented, and the implications for studies of Mg discussed. The propagation of uncertainties in the inelastic collision data to those in stellar abundances is investigated, and found to lead to small uncertainties, once again typically less than 0.01 dex (2%), although for few stellar models in specific lines (e.g., metal-poor suns, in the 7691 A line) uncertainties can be as large as 0.03 dex (7%).

An Analysis of Low Frequency Discharge in a CH3SiCl3-Ar-H2 Mixture by Optical Emission Spectroscopy and Actinometry

Low frequency (100 kHz) discharge in Ar-H2 and CH3SiCl3-Ar-H2 mixtures was studied to obtain information on the processes involved in plasma deposition of SixCy:H films from CH3SiCl3-Ar-H2 plasma once the properties of Ar-H2 plasma are known. The plasmas were studied using optical emission spectroscopy. The addition of small amounts of nitrogen to the plasma mixtures also permitted the use of an actinometry technique. First, plasma parameters (electron density and temperature) and actinometric concentrations of atomic hydrogen in an argon–hydrogen plasma were investigated as a function of the hydrogen content in the feed. Second, the emission intensities of Si, Si+, CH, H, Ar and Ar+ species produced in the CH3SiCl3-Ar-H2 discharge were analysed as a function of time following the introduction of CH3SiCl3 (methyltrichlorosilane, MTCS) to the argon–hydrogen plasmas with various proportions of the feed gasses. The results reveal a rapid decay of the Si-excited state number density versus time, while those of Si+ and CH fell off more slowly. The emission of atomic silicon was believed to be a result of electron impact dissociative and excitation processes occurring in the bulk of the discharge, whereas the Si+ and CH seemed to originate mainly from products of sputtering of the growing film surface. The fragmentation of the MTCS associated with HCl formation and enhanced atomic hydrogen production as a result of HCl dissociation are proposed. Variations in the radical densities of H and CH3 were determined using an actinometry technique. The results indicate a significant role for H2 in gas-phase reactions occurring in the CH3SiCl3-Ar-H2 plasma, as well as in gas-surface interactions, leading to competition between deposition and chemical sputtering of already deposited material.

Cross section and rate coefficient calculations for electron impact excitation of the a $^{\sf 3}{\sf \Pi}$ , a’ $^{\sf 3}{\sf \Sigma^{+}}$ , d $^{\sf 3}{\sf \Delta}$ , e $^{\sf 3}{\sf \Sigma^{-}}$ , I $^{\sf 1}{\sf \Sigma^{+}}$ and D $^{\sf 1}{\sf \Delta}$ states of CO

An efficient theoretical method for investigating the cross sections and rate coefficients of electron impact excitation processes involving carbon monoxide (CO) diatomic molecules has been applied in the temperature range 1500–15 000 K. The weighted total cross section (WTCS) theory has been used to investigate the cross sections excitation of the a 3Π, a’ 3Σ+, d 3Δ, e 3Σ-, I 1Σ+ and D 1Δ states of CO. The 300 K cross sections as a function of electron energy and one temperature rate coefficients results are presented and fitting parameters (a, b and c) are given for each reaction rate coefficient: k(θ) = a (θb) exp (-c/θ).

Electron-impact excitation and ionization of complex atoms

Taking advantage of significant improvements in multi-hit detector technology and algorithm development, together with the rapid increase of computational power, significant advances have recently been reported in the experimental and theoretical studies of electron-impact excitation and ionization processes. The current state-of-the-art in this field is illustrated by a few key examples.