Electron-impact Excitation Cross Sections Research Articles

Ratios of double to single ionization of He and Ne by strong 400-nm laser pulses using the quantitative rescattering theory

We present numerical simulations of the ratio between double and single ionization of He and Ne by intense laser pulses at wavelengths of 390 and 400 nm, respectively. The yields of doubly charged ions due to nonsequential double ionization (NSDI) are obtained by employing the quantitative rescattering (QRS) model. In this model, the NSDI ionization probability is expressed as a product of the returning electron wave packet (RWP) and the total scattering cross sections for laser-free electron impact excitation and electron impact ionization of the parent ion. According to the QRS theory, the same RWP is also responsible for the emission of high-energy above-threshold ionization photoelectrons. To obtain absolute double-ionization yields, the RWP is generated by solving the time-dependent Schr\odinger equation (TDSE) within a one-electron model. The same TDSE results can also be taken to obtain single-ionization yields. By using the TDSE results to calibrate single ionization and the RWP obtained from the strong-field approximation, we further simplify the calculation such that the nonuniform laser intensity distribution in the focused laser beam can be accounted for. In addition, laser-free electron impact excitation and ionization cross sections are calculated using the state-of-the-art many-electron $R$-matrix theory. The simulation results for double-to-single-ionization ratios are found to compare well with experimental data and support the validity of the nonsequential double-ionization mechanism for the covered intensity region.

Analysis of Data on the Cross Sections for Electron-Impact Ionization and Excitation of Electronic States of Atomic Hydrogen (Review)

A review is given of experimental and theoretical data on the cross sections for ionization, excitation, and deexcitation of atomic hydrogen. The set of the cross sections required to calculate the electron energy distribution function and find the level-to-level rate coefficients needed to solve balance equations for the densities of neutral and charged particles in hydrogen plasma is determined.

The energy structure and decay channels of the 4p6-shell excited states in Sr

The ejected-electron spectra arising from the decay of the 4p autoionizing states in Sr atoms have been studied precisely at the incident-electron energies close to excitation and ionization thresholds of the subshell. The excitation behaviors for 58 lines observed between 12 and 21 eV ejected-electron kinetic energy have been investigated. Also, the ab initio calculations of excitation energies, autoionization probabilities and electron-impact excitation cross sections of the states 4p (nl = 4d, 5s, 5p; = 4d, 5s, 5p; = 5s, 6s, 7s, 8s, 9s, 5p, 6p, 5d, 6d, 7d, 8d, 4f, 5g) have been performed by employing the large-scale configuration-interaction method in the basis of the solutions of Dirac–Fock–Slater equations. The obtained experimental and theoretical data have been used for the accurate identification of the 60 lines in ejected-electron spectra and the 68 lines observed earlier in photoabsorption spectra. The excitation and decay processes for 105 classified states in the 4p55s, 4p54d and 4p55s configurations have been considered in detail. In particular, most of the states lying below the ionization threshold of the 4p6 subshell at 26.92 eV possess up to four decay channels with formation of Sr+ in 5s, 4d and 5p states. Two-step autoionization and two-electron Auger transitions with formation of Sr2+ in the 4p6 ground state are the main decay paths for high-lying autoionizing states. The excitation threshold of the subshell in Sr has been established at 20.98 ± 0.05 eV.

Electron-impact vibrational excitation of the hydroxyl radical in the nighttime upper atmosphere

Chemical processes produce vibrationally excited hydroxyl (OH) in a layer centred at an altitude of about 87 km in the Earth's atmosphere. Observations of this layer are used to deduce temperatures in the mesosphere and to observe the passage of atmospheric gravity waves. Due to the low densities and energies at night of electrons at the relevant altitude, it is not expected that electron-impact excitation of OH would be significant. However, there are unexplained characteristics of OH densities and radiative emissions that might be explained by electron impact. These are measurements of higher than expected densities of OH above 90 km and of emissions at higher energies that cannot be explained by the chemical production processes. This study simulates the role of electron impact in these processes, using theoretical cross sections for electron-impact excitation of OH. The simulations show that electron impact, even in a substantial aurora, cannot fully explain these phenomena. However, in the process of this investigation, apparent inconsistencies in the theoretical cross sections and reaction rates were found, indicating that measurements of electron-impact excitation of OH are needed to resolve these problems and scale the theoretical predictions to allow more accurate simulations.

Relativistic R -matrix calculations for the electron-impact excitation of neutral molybdenum

A recent PISCES-B Mod experiment [Nishijima et al., J. Phys. B 43, 225701 (2010)] has revealed up to a factor of 5 discrepancy between measurement and the two existing theoretical models [Badnell et al., J. Phys. B 29, 3683 (1996); Bartschat et al., J. Phys. B 35, 2899 (2002)], providing important diagnostics for Mo i. In the following paper we address this issue by employing a relativistic atomic structure and $R$-matrix scattering calculations to improve upon the available models for future applications and benchmark results against a recent Compact Toroidal Hybrid experiment [Hartwell et al., Fusion Sci. Technol. 72, 76 (2017)]. We determine the atomic structure of Mo i using grasp${}^{0}$, which implements the multiconfigurational Dirac-Fock method. Fine structure energies and radiative transition rates are presented and compared to existing experimental and theoretical values. The electron-impact excitation of Mo i is investigated using the relativistic $R$-matrix method and the parallel versions of the Dirac atomic $R$-matrix codes. Electron-impact excitation cross sections are presented and compared to the few available theoretical cross sections. Throughout, our emphasis is on improving the results for the $z\phantom{\rule{0.16em}{0ex}}^{5}P_{1,2,3}^{o}\ensuremath{\rightarrow}a\phantom{\rule{0.16em}{0ex}}^{5}S_{2},\phantom{\rule{0.16em}{0ex}}z\phantom{\rule{0.16em}{0ex}}^{7}P_{2,3,4}^{o}\ensuremath{\rightarrow}a\phantom{\rule{0.16em}{0ex}}^{7}S_{3}$ and $y\phantom{\rule{0.16em}{0ex}}^{7}P_{2,3,4}^{o}\ensuremath{\rightarrow}a\phantom{\rule{0.16em}{0ex}}^{7}S_{3}$ electric dipole transitions of particular relevance for diagnostic work.

Photoionization and electron impact excitation cross sections for Fe I

Context. Iron is a major contributor to the opacity in the atmospheres of late-type stars, as well as a major contributor to the observed lines in their visible spectrum. Iron lines are commonly used to derive basic stellar parameters from medium/high resolution spectroscopy, that is, spectroscopy which shows metal content, effective temperature, and surface gravity. Aims. Here we present large R-matrix calculations for photoionization cross sections and electron impact collision strengths.Methods. The photoionization calculations included 35 configurations and 134 LS close coupling terms of the target ion. The eigenfunction expansion accounts for the photoionization of the outer nl subshells, with n ≥ 4, as well as the open inner 3d subshell. Our results include total and partial (term-to-term) photoionization cross sections for 936 energy terms of iron with principal quantum number ≤10, and total angular momentum from zero to seven. Our electron impact collision strengths include the lowest 46 LS terms of the atom.Results. The present photoionization cross sections should be considerably more accurate than those currently available in the literature. On the other hand, the electron impact cross sections, which are being reported for the first time, are needed in non-local thermodynamic equilibrium (NLTE) modeling of the solar spectrum and late-type stars in general.

Experimental scaling of plane-Born cross sections for the electron-impact excitation to the b1Πu state of N2 molecule

SynopsisWe report measurements on differential cross sections (DCSs) for electron-impact excitation to the optically-allowed b1Πu state in molecular nitrogen in the impact energy range of 20 – 400 eV. Absolute excitation DCSs normalized by the relative flow technique were transformed to generalized oscillator strengths (GOSs) as a function of momentum transfer squared. Integral cross sections (ICSs) for the transitions to the b1Πu state were compared with the BEf-scaled plane-Born ICS as well as previous experimental studies.

The influence of Breit interaction and E1-M2 quantum interference on polarization following inner-shell electron-impact excitation of Li-like ions

SynopsisThe total and magnetic sublevel cross sections of electron-impact excitation from the ground state 1s22s 2S1/2 to the inner-shell excited states 1s2s2p (J=3/2) are calculated based on a relativistic distorted-wave method for highly charged Li-like ions. The important influence of the Breit interactions and quantum interferences between electronic dipole (E1) and magnetic quadrupole (M2) transition on polarization of emitted X-ray are systematically studied.

A project based on multi-configuration Dirac–Fock calculations for plasma spectroscopy

We present a project dedicated to hot plasma spectroscopy based on a Multi-Configuration Dirac–Fock (MCDF) code, initially developed by J. Bruneau. The code is briefly described and the use of the transition state method for plasma spectroscopy is detailed. Then an opacity code for local-thermodynamic-equilibrium plasmas using MCDF data, named OPAMCDF, is presented. Transition arrays for which the number of lines is too large to be handled in a Detailed Line Accounting (DLA) calculation can be modeled within the Partially Resolved Transition Array method or using the Unresolved Transition Arrays formalism in jj-coupling. An improvement of the original Partially Resolved Transition Array method is presented which gives a better agreement with DLA computations. Comparisons with some absorption and emission experimental spectra are shown. Finally, the capability of the MCDF code to compute atomic data required for collisional-radiative modeling of plasma at non local thermodynamic equilibrium is illustrated. In addition to photoexcitation, this code can be used to calculate photoionization, electron impact excitation and ionization cross-sections as well as autoionization rates in the Distorted-Wave or Close Coupling approximations. Comparisons with cross-sections and rates available in the literature are discussed.

Theoretical resonant electron-impact vibrational excitation, dissociative recombination and dissociative excitation cross sections of ro-vibrationally excited BeH+ ion

A theoretical study of resonant vibrational excitation, dissociative recombination and dissociative excitation processes of the beryllium monohydride cation, BeH+, induced by electron impact, is reported. Full sets of ro-vibrationally-resolved cross sections and of the corresponding Maxwellian rate coefficients are presented for the three processes. Particular emphasis is given to the high-energy behaviour. Potential curves of , and symmetries and the corresponding resonance widths, obtained from R-matrix calculations, provide the input for calculations which use a local complex-potential model for resonant collisions in each of the three symmetries. Rotational motion of nuclei and isotopic effects are also discussed. The relevant results are compared with those obtained using a multichannel quantum defect theory method. Full results are available from the Phys4Entry database.

Electron-impact excitation cross-sections for high-lying even levels of the dysprosium atom

Electron-impact excitation cross-sections for high-lying even levels of the dysprosium atom

Calculation of fully relativistic cross sections for electron excitation of cesium atom and its application to the diagnostics of hydrogen-cesium plasma

Electron impact excitation cross-sections and rate coefficients have been calculated using fully relativistic distorted wave theory for several fine-structure transitions from the ground as well as excited states of cesium atom in the wide range of incident electron energy. These processes play dominant role in low pressure hydrogen-cesium plasma, which is relevant to the negative ion based neutral beam injectors for the ITER project. As an application, the calculated detailed cross-sections are used to construct a reliable collisional radiative (CR) model to characterize the hydrogen-cesium plasma. Other processes such as radiative population transfer, electron impact ionization and mutual neutralization of Cs+ ion with negative hydrogen ion along with their reverse processes are also taken into account. The calculated cross-sections and the extracted plasma parameters from the present model are compared with the available experimental and theoretical results.

Calculated low-energy electron-impact vibrational excitation cross sections for CO2 molecule

Vibrational-excitation cross sections of ground electronic states of a carbon dioxide molecule by electron-impact through CO shape resonance is considered in the separation of the normal modes approximation. Resonance curves and widths are computed for each vibrational mode. The calculations assume a decoupling between normal modes and employ the local complex potential model for the treatment of nuclear dynamics, usually adopted for electron-scattering involving diatomic molecules. Results are presented for excitation up to 10 vibrational levels in each mode and a comparison with data present in the literature is discussed.

Spin and spatial dynamics in electron-impact scattering offS-wave He usingR-matrix-with-time-dependence theory

R-matrix with time-dependence theory is applied to electron-impact ionisation processes for He in the S-wave model. Cross sections for electron-impact excitation, ionisation and ionisation with excitation for impact energies between 25 and 225 eV are in excellent agreement with benchmark cross sections. Ultra-fast dynamics induced by a scattering event is observed through time-dependent signatures associated with autoionisation from doubly excited states. Further insight into dynamics can be obtained through examination of the spin components of the time-dependent wavefunction.

Absolute cross sections for electronic excitation of pyrimidine by electron impact.

We measured differential cross sections for electron-impact electronic excitation of pyrimidine, both as a function of electron energy up to 18 eV, and of scattering angle up to 180°. The emphasis of the present work is on recording detailed excitation functions revealing resonances in the excitation process. The differential cross sections were summed to obtain integral cross sections. These are compared to results of R-matrix calculations, which successfully reproduce both the magnitude of the cross section and the major resonant features. Comparison of the experiment to the calculated contributions of different symmetries to the integral cross section permitted assignment of several features to specific core-excited resonances. Comparison of the resonant structure of pyrimidine with that of benzene revealed pronounced similarities and thus a dominant role of π-π(∗) excited states and resonances. Electron energy loss spectra were measured as a preparation for the cross section measurements and vibrational structure was observed for some of the triplet states. A detailed analysis of the electronic excited states of pyrimidine is also presented.

Excitation of vibrational quanta in furfural by intermediate-energy electrons.

We report cross sections for electron-impact excitation of vibrational quanta in furfural, at intermediate incident electron energies (20, 30, and 40 eV). The present differential cross sections are measured over the scattered electron angular range 10°-90°, with corresponding integral cross sections subsequently being determined. Furfural is a viable plant-derived alternative to petrochemicals, being produced via low-temperature plasma treatment of biomass. Current yields, however, need to be significantly improved, possibly through modelling, with the present cross sections being an important component of such simulations. To the best of our knowledge, there are no other cross sections for vibrational excitation of furfural available in the literature, so the present data are valuable for this important molecule.

Electron Impact Excitation and Dielectronic Recombination of Highly Charged Tungsten Ions

Electron impact excitation (EIE) and dielectronic recombination (DR) of tungsten ions are basic atomic processes in nuclear fusion plasmas of the International Thermonuclear Experimental Reactor (ITER) tokamak. Detailed investigation of such processes is essential for modeling and diagnosing future fusion experiments performed on the ITER. In the present work, we studied total and partial electron-impact excitation (EIE) and DR cross-sections of highly charged tungsten ions by using the multiconfiguration Dirac–Fock method. The degrees of linear polarization of the subsequent X-ray emissions from unequally-populated magnetic sub-levels of these ions were estimated. It is found that the degrees of linear polarization of the same transition lines, but populated respectively by the EIE and DR processes, are very different, which makes diagnosis of the formation mechanism of X-ray emissions possible. In addition, with the help of the flexible atomic code on the basis of the relativistic configuration interaction method, DR rate coefficients of highly charged W37+ to W46+ ions are also studied, because of the importance in the ionization equilibrium of tungsten plasmas under running conditions of the ITER.

Laser-assisted inelastic scattering of electrons by helium atoms

The differential cross section for electron-impact excitation of helium atoms in the presence of a linearly polarized laser field is calculated. The interaction of the laser field with both the projectile electron and the target atom is treated in a fully nonperturbative way, while the electron-atom interaction is treated within the first Born approximation. We are interested in studying two cases where the photon energy of the laser field is chosen to be far from resonance and when it matches with $2{\phantom{\rule{0.16em}{0ex}}}^{1}S\ensuremath{\longrightarrow}2{\phantom{\rule{0.16em}{0ex}}}^{1}P$ and $3{\phantom{\rule{0.16em}{0ex}}}^{1}S\ensuremath{\longrightarrow}3{\phantom{\rule{0.16em}{0ex}}}^{1}P$ transition frequencies. The agreement between perturbative and nonperturbative results is good, except close to the resonance where the perturbative cross sections diverge while the nonperturbative ones predict no maximum of the cross sections. Another interesting effect is the presence of an avoided crossing of the Floquet pseudoenergies at resonance.

Influence of Breit interaction on the cross section and circular polarization of radiation emitted by longitudinally-polarized electron impact excitation of highly charged Be-like ions

SynopsisWe investigated the contributions of the Breit interaction to the longitudinally-polarized electron impact excitation cross sections and circular polarizations of the 1s2s22p3/2(J = 2) → 1s22s2(J = 0) line of highly charged Be-like ions using a fully relativistic distorted-wave method. It is found that the Breit interaction can modify the Mf = -1 and -2 sublevels cross sections by several orders of magnitude. These dramatic influences also lead to a remarkable decrease in the circular polarization of radiation, which characters becomes more evident as the incident energy and/or atomic number increases. And all these features are different from the conclusions for the linear polarization of radiation.

Cascades effects on the polarization properties of x-ray radiation emitted by electron-impact excitation of highly charged He-like Fe24+ ions

SynopsisDetailed calculations using a fully relativistic distorted-wave method are carried out for the electron-impact excitation cross sections from the ground state 1s2 (J= 0) to the individual magnetic sublevels of the excited state 1s2s1/2 (J= 1) of highly charged He-like Fe24+ ions. The cascades effects to the cross sections and the linear polarizations of the 1s2s1/2(J= 1) → 1s2 (J=0) line are investigated systematically. Our results show that the 1s2p, 1s3l, and 1s4l (l = s, p, d) excited states have significant effects and make the cross sections increase. These influences also lead to a remarkable decrease in the linear polarization of subsequent x-ray radiation. The present results agree with the previous theoretical results and the polarization measurements very well.