Dielectronic Recombination Research Articles

X-ray spectra of the Fe-L complex

The Hitomi results on the Perseus cluster have led to improvements in our knowledge of atomic physics that are crucial for the precise diagnostic of hot astrophysical plasma observed with high-resolution X-ray spectrometers. However, modeling uncertainties remains, both within but especially beyond Hitomi’s spectral window. A major challenge in spectral modeling is the Fe-L spectrum, which is basically a complex assembly ofn ≥ 3 ton = 2 transitions of Fe ions in different ionization states, affected by a range of atomic processes such as collisional excitation, resonant excitation, radiative recombination, dielectronic recombination, and innershell ionization. In this paper we perform a large-scale theoretical calculation on each of the processes with the flexible atomic code (FAC), focusing on ions of Fe XVIIto Fe XXIVthat form the main body of the Fe-L complex. The calculation includes a large set of energy levels with a broad range of quantum numbernandl, taking into account the full-order configuration interaction and all possible resonant channels between two neighboring ions. The new data are found to be consistent within 20% with the recent individualR-matrix calculations for the main Fe-L lines, although the discrepancies become significantly larger for the weaker transitions, in particular for Fe XVIII, Fe XIX, and Fe XX. By further testing the new FAC calculations with the high-quality RGS data from 15 elliptical galaxies and galaxy clusters, we note that the new model gives systematically better fits than the current SPEX v3.04 code, and the mean Fe abundance decreases by 12%, while the O/Fe ratio increases by 16% compared with the results from the current code. Comparing the FAC fit results to those with theR-matrix calculations, we find a temperature-dependent discrepancy of up to ∼10% on the Fe abundance between the two theoretical models. Further dedicated tests with both observed spectra and targeted laboratory measurements are needed to resolve the discrepancies, and ultimately to get the atomic data ready for the next high-resolution X-ray spectroscopy mission.

Investigation of the intershell higher order resonant recombination and polarization of X‐ray emission of B‐like silicon ions

K‐shell dielectronic, trielectronic, and quadruelectronic recombination and polarization of X‐ray emission have been studied for the highly charged Si9+ ions in the initial ground state 2P1/2 and the metastable state 2P3/2 using the Flexible Atomic Code. It is found that the resonant recombination cross section from the long‐lived metastable state is comparable in magnitude with that of the ground state, so it is important for plasma diagnostics. For Si9+(2P1/2), trielectronic recombination contributions of nearly 25% to the total resonant recombination strength are predicted, which is less than previous calculations. We compare the degree of linear polarization for the eleven dominant resonant recombination satellite lines from the initial parent Si9+(2P1/2) and Si9+(2P3/2) ions. For the same X‐ray lines, large variations of polarization are found between 2P1/2 and 2P3/2, which can be employed to diagnose formation mechanisms of intermediate resonance states and corresponding X‐ray lines.

Modelling ion populations in astrophysical plasmas: carbon in the solar transition region

The aim of this work is to improve the modelling of ion populations in higher density, lower temperature astrophysical plasmas, of the type commonly found in lower solar and stellar atmospheres. Ion population models for these regions frequently employ the coronal approximation, which assumes conditions more suitable to the upper solar atmosphere, where high temperatures and lower densities prevail. The assumed conditions include all ions being in the ground state and steady-state equilibrium, where there is sufficient time for ionisation and recombination to take place. Using the coronal approximation for modelling the solar transition region gives theoretical lines intensities for the Li-like and Na-like isoelectronic sequences which are often factors of two to five times lower than observed. The works of Burgess & Summers (1969, ApJ, 157, 1007) and Nussbaumer & Storey (1975, A&A, 44, 321) show the important part ions in excited levels play when included in the modelling. As density increases metastable levels become populated and ionisation rates increase, whereas dielectronic recombination through highly excited levels is suppressed. Photo-ionisation is also shown by Nussbaumer & Storey to have an effect on the charge-state distribution of carbon in these regions. Their models, however, use approximations for the atomic rates to determine the ion balance. Presented here is the first stage in updating these earlier models of carbon by using rates from up-to-date atomic calculations and more recent photo-ionising radiances. Where atomic rates were not readily available, in the case of electron impact direct ionisation and excitation–auto-ionisation, new calculations were made using the Flexible Atomic Code and Autostructure, and compared to theoretical and experimental studies. The effects each atomic process has on the ion populations as density changes is illustrated, and final results from the modelling are compared to the earlier works. Lastly, the new results for ion populations were used to predict line intensities for the solar transition region in the quiet Sun. In comparison to coronal approximation modelling the new results show significantly improved agreement with observations.

Dielectronic recombination at low energy range with Boron‐like Ar13+at the CSRm

In order to extend the dielectronic recombination (DR) experiments from the main cooler storage ring CSRm with intermediate charge state ions to the experimental cooler storage ring CSRe with highly charged ions and even radioactive ions, a test electron‐ion recombination experiment of B‐like Ar13+ ions was performed with a section of the new electron beam energy detuning system, which will be installed at the CSRe. We present the measured and also the calculated DR spectra for the ∆n = 0 resonances from 0 to 3 eV. The experimental results agree very well with the previous DR experimental data from CRYRING, and the energy resolution reached 30 meV full width at half maximum. This test experiment demonstrates that the reliability and stability of the newly developed electron beam energy detuning system are satisfactory for the upcoming DR experiments at the CSRe. However, we found large discrepancies between the experimental result and the calculation in both resonance positions and the intensities of the recombination spectrum below 0.7 eV, which indicate that precise calculation of the DR spectrum of multielectron ions is still a challenge.

Ab initio dielectronic recombination rate coefficients for highly-charged Ar-like ions

Employing the independent-processes isolated-resonance distorted-wave approximation, we have made a comprehensive study on dielectronic recombination (DR) for 19 highly-charged Ar-like ions with nuclear charge number Z=23–30, 36, 42, 50, 54, 60, 64, 70, 74, 79, 85 and 92. For the convenience of usage, the ab initio level-by-level total rate coefficients for DR initially from both the ground state 3s23p6 1S0 and the metastable state 3s23p53d 3P0 are presented in a simple compact fitting form. We include the 3 → 3 (Δn=0), 2 → 3 and 3 → 4 (Δn=1), and 3 → 5 (Δn=2) core-excitations resonances as well as the decays from the resonances toward low-lying autoionizing levels followed possibly by radiative cascade effects in our ab initio DR results. The detailed configuration interaction analysis and comparisons with both theoretical and experimental results show that present total DR rate coefficients are reliable at both low (with a typical uncertainty of a factor of 2 in the photoionized plasma zone) and high (with an uncertainty of about 20% in the collisional plasma zone) temperature. To our knowledge, these are the first ab initio DR results for most of ions with Z > 30.

CHIANTI—An Atomic Database for Emission Lines. XV. Version 9, Improvements for the X-Ray Satellite Lines

Abstract CHIANTI contains a large quantity of atomic data for the analysis of astrophysical spectra. Programs are available in IDL and Python to perform calculation of the expected emergent spectrum from these sources. The database includes atomic energy levels, wavelengths, radiative transition probabilities, rate coefficients for collisional excitation, ionization, and recombination, as well as data to calculate free–free, free–bound, and two-photon continuum emission. In Version 9, we improve the modeling of the satellite lines at X-ray wavelengths by explicitly including autoionization and dielectronic recombination processes in the calculation of level populations for select members of the lithium isoelectronic sequence and Fe xviii–xxiii. In addition, existing data sets are updated, new ions are added, and new total recombination rates for several Fe ions are included. All data and IDL programs are freely available at http://www.chiantidatabase.org or through SolarSoft, and the Python code ChiantiPy is also freely available at https://github.com/chianti-atomic/ChiantiPy.

EBIT Observation of Ar Dielectronic Recombination Lines near the Unknown Faint X-Ray Feature Found in the Stacked Spectrum of Galaxy Clusters.

Motivated by possible atomic origins of the unidentified emission line detected at 3.55-3.57 keV in a stacked spectrum of galaxy clusters, an electron beam ion trap (EBIT) was used to investigate the resonant dielectronic recombination (DR) process in highly charged argon ions as a possible contributor to the emission feature. The He-like Ar DR-induced transition 1s22l-1s2l3l' was suggested to produce a 3.62 keV photon near the unidentified line at 3.57 keV and was the starting point of our investigation. The collisional-radiative model NOMAD was used to create synthetic spectra for comparison with both our EBIT measurements and with spectra produced with the AtomDB database/Astrophysical Plasma Emission Code (APEC) used in the Bulbul et al. work. Excellent agreement was found between the NOMAD and EBIT spectra, providing a high level of confidence in the atomic data used. Comparison of the NOMAD and APEC spectra revealed a number of missing features in the AtomDB database near the unidentified line. At an electron temperature of T e = 1.72 keV, the inclusion of the missing lines in AtomDB increases the total flux in the 3.5-3.66 keV energy band by a factor of 2. While important, this extra emission is not enough to explain the unidentified line found in the galaxy cluster spectra.

State-selective quantum interference studied in the photo-recombination of Ar17+ ion

We investigate the total and partial cross sections of the state-selective photo-recombination (PR) from the ground state 1sS01 of H-like Ar17+ ion to the 1s2S01, 1s2sS01,S13 and 1s2pP0,1,23,P11 states of He-like Ar16+ ion. The analysis is performed based on the close-coupling approximation and the fully relativistic Dirac R-matrix method, in which the unified treatments of the combined electron-ion PR process, including radiative and dielectronic recombination (DR) as coherent, interfering components are considered. The KLL PR resonance group are determined and identified according to the calculated energies and transition rates using the multiconfiguration Dirac–Fock (MCDF) method. The DR cross sections are also presented for comparison purposes. Our results show that the quantum interference between direct recombination and resonant recombination processes will strongly influence the intensity and shape of the PR cross sections, especially for the 2s2pP11 and 2p2D21 resonant regions. Comparison of our computed results with previously published experimental and theoretical values, when available, is made.

Resonance strengths for KLL dielectronic recombination of highly charged mercury ions and improved empirical Z -scaling law

Theoretical and experimental resonance strengths for KLL dielectronic recombination (DR) into He-, Li-, Be-, and B-like mercury ions are presented, based on state-resolved DR x-ray spectra recorded at the Heidelberg electron beam ion trap. The DR resonance strengths were experimentally extracted by normalizing them to simultaneously recorded radiative recombination signals. The results are compared to state-of-the-art atomic calculations that include relativistic electron-electron correlation and configuration mixing effects. Combining the present data with other existing ones, we derive an improved semi-empirical $Z$-scaling law for DR resonance strength as a function of the atomic number, taking into account higher-order relativistic corrections, which are especially relevant for heavy highly charged ions.

Near-threshold excitation of the λ132.2 nm resonance line of the Tl<sup>+</sup> ion by electron impact

Background : Among all processes of collision of electrons with atomic particles the study of the interaction between electrons and ions is extremely important and necessary for a better understanding of the basic collisional processes in plasma. In the processes of excitation, recombination, and ionization of ions by electron impact autoionization states play an important role. Their radiation decay makes an additional contribution to the photo-recombination process and leads to dielectronic recombination of an ion. Such contribution is most significant in the case of heavy many-electron ions such as the Tl + ion. Excitation of the Tl + ion by an electron impact at electron-ion collisions is practically not investigated up to date. Methods: Experiments were performed using a spectroscopic method in conditions of electron and ion beams intersecting at an angle of 90°. The ribbon electron beam (cross section 1×8 mm 2 , energy E e = 6–14 eV, current I e = (2–10)×10 –5 A) was formed by a low-energy three-electrode electron gun. The ion beam (cross section 2.5×2.5 mm 2 , energy E i = 800 eV, current I i = (5–7)×10 –7 A) was obtained using a low-voltage arc discharge source. The radiation due to decay of the ion states excited during the collisions was detected perpendicular to the plane of the beams intersection. Spectral separation of the radiation in a vacuum ultraviolet region was carried out by 70° vacuum monochromator based on the Seya-Namioka scheme. Results: Thorough measurements of the near-threshold region of the energy dependence of the effective cross section of the electron excitation of the resonance l 132.2 nm line (6 s 6 p 1 P o 1 → 6 s 2 1 S 0 ) of the Tl + ion in the 6–14 eV energy range revealed resonant features both after and before the excitation threshold of the line. The widths of the before-threshold peculiarities is defined by the dispersion of the excited electrons energy. This is indicative of their resonant nature, i.e., they are due to the formation and decay of the atoionization states (AIS) of the Tl atom. Most likely, they are related to the formation and decay of the AIS of the 6 s 6 pnp configuration ( n ≥ 7) to the excited levels of the Tl atom. The nature of the after-threshold structure indicates that it is also the result of resonance processes and is due to the formation and decay of the AIS, most likely of the 5 d 9 6 s 2 6 p 2 configuration, in the electronic channel. The absolute values of the excitation cross sections of the dielectronic satellites were determined by comparison of their intensities with the intensity of the resonance l 132.2 nm line which effective excitation cross section was obtained by normalizing the experimental data on the theoretical calculation by the tight-binding method of two states of the excitation cross section of the 6 s 6 p 1 P o 1 level of the Tl + ion at the 100 eV energy. Conclusions: The energy dependence of the effective excitation cross section of the dielectronic satellites of the Tl + ion resonance line showed that the absolute values of their excitation cross sections are ~ 10 –16 cm 2 and are of the same order of magnitude with that for the resonance line. It is shown that the main excitation mechanism of the resonance line satellites is dielectronic recombination which effectiveness essentially depends on the ratio of the probabilities of the AIS radiation and electron decay. In the case of the relativistic Tl + ion a strong configuration mixing of levels leads to significant increase of the AIS radiation decay probability.

Laboratory Measurements of X-Ray Emission from Highly Charged Argon Ions

Abstract Uncertainties in atomic models will introduce noticeable additional systematics in calculating the flux of weak dielectronic recombination (DR) satellite lines, affecting the detection and flux measurements of other weak spectral lines. One important example is the Ar xvii Heβ DR, which is expected to be present in emission from the hot intracluster medium of galaxy clusters and could impact measurements of the flux of the 3.5 keV line that has been suggested as a secondary emission from a dark matter interaction. We perform a set of experiments using the Lawrence Livermore National Laboratory’s electron beam ion trap (EBIT-I) and the X-ray Spectrometer quantum calorimeter (XRS/EBIT) to test the Ar xvii Heβ DR origin of the 3.5 keV line. We measured the X-ray emission following resonant DR onto helium-like and lithium-like Argon using EBIT-I’s Maxwellian simulator mode at a simulated electron temperature of T e = 1.74 keV. The measured flux of the Ar xvii Heβ DR lined is too weak to account for the flux in the 3.5 keV line, assuming reasonable plasma parameters. We, therefore, rule out Ar xvii Heβ DR as a significant contributor to the 3.5 keV line. A comprehensive comparison between the atomic theory and the EBIT experiment results is also provided.

KLL Dielectronic Satellite Spectra from the Photorecombination of He-like Fe and Ni Ions

Abstract The radiative decay of a doubly excited autoionizing state is characterized by the dielectronic recombination in the photorecombination (PR) process. We calculate PR cross sections for the ground states of the Fe xxv and Ni xxvii ions by employing the Dirac R-matrix technique. The autoionizing states are treated in the framework of Davies and Seaton’s theory to account for the radiative decay. The strengths of the KLL resonances that equate directly to the dielectronic satellite line strengths are evaluated. Besides, the Multiconfiguration Dirac Fock formalism is utilized to obtain the dielectronic satellite line strengths under the isolated resonance approximation. A very good agreement is found for the wavelengths and transition parameters belonging to the KLL autoionizing states with the previous theoretical and experimental results. The dominant KLL dielectronic satellite line strengths from both theoretical models are consistent with each other and are in good agreement with the established theoretical models and measurements at the electron beam ion trap.

Linear polarization of the dielectronic recombination Kα satellite lines in Li-like [formula omitted

The degree of linear polarization of the dielectronic recombination Kα satellite lines in Li-like Au76+ is obtained within the density matrix formalism based on the relativistic atomic data computed via relativistic distorted wave approximation. In the absence of accurate laboratory wavelength measurements for the K-shell resonance lines in gold ions, the atomic data for the He-like Au77+ target ion were computed in a systematic approach and compared with existing semi-relativistic Breit-Pauli calculations [S. Nahar, A.K. Pradhan, C. Sur, JQSRT 2008;109:1951]. The computation for the recombined Au76+ ion comprises 915 fine-structure levels arising from 1s2nl, 1s2snl, 1s2pnl, and 1s3l3l′ configurations, with n ≤ 9 and l ≤ 4, l′ ≤ 2, which yield 731 Kα transitions of type 1s2pnl−1s2nl within 0.17486 to 0.18776 Åspectral range. The degree of linear polarization for the overall Kα satellites as well as for each individual line is computed. Present results show how radiation from neighboring resonances gives peaks of local minima or maxima in the polarization when plotted against the kinetic energy of the incoming electron. Interference effects occurring for bright lines emitted from spatially close resonances result in depolarization of the radiation and displacement of the polarization peaks. Present quantitative results are useful for gold plasma diagnostics and fundamental physics .

Theoretical investigation of electron-ion recombination processes of Li-like tungsten ions

Dielectronic recombination (DR) and radiative recombination (RR) spectra of highly charged Tungsten ions are important for modeling and diagnosing magnetic fusion plasmas. We studied the DR process of Li-like W71+(2s) ions using the flexible atomic code (FAC) based on the relativistic configuration interaction (RCI) method with Breit and QED corrections. The detailed resonance energies, widths and strengths were calculated systematically for the dominated doubly excited states (2p1/2nlj)J(n=19∼29) and (2p3/2n′lj)J(n′=7∼29) of Be-like W70+ ions. The contributions from the higher Rydberg states with n=30∼ncut(ncut=116) are obtained by extrapolation based on the quantum defect theory (QDT). The electron-ion recombination spectra (DR+RR), covering the center-of-mass energy range 0–1700 eV, are presented by taking into account the electron beam temperature at the experimental cooler storage ring (CSRe). It is found that the DR resonances associated with 2s1/2 → 2p1/2 and 2s1/2 → 2p3/2 transitions are not blending for the whole Δn=0 collision energy range, which will provide a good chance for testing fundamental theory clearly by experimental investigation. Finally the plasma rate coefficients are deduced from the calculated recombination spectra in the temperature range from 5.0 × 103 to 1.0 × 108 K. Compared with the AUTOSTUCTURE results, a good agreement is obtained except for the DR rates at the low electron temperature (Te < 5.0 × 105K), which were mainly attributed to the difference of the first resonance group 2p1/219l. The present results will guide the DR experiment of Tungsten ions at the CSRe and also be useful for ITER fusion plasmas modeling.

Suppression of Dielectronic Recombination Due to Finite Density Effects. II. Analytical Refinement and Application to Density-dependent Ionization Balances and AGN Broad-line Emission

We present improved fits to our treatment of suppression of dielectronic recombination at intermediate densities. At low densities, most recombined excited states eventually decay to the ground state, and therefore the total dielectronic recombination rate to all levels is preserved. At intermediate densities, on the other hand, collisions can lead to ionization of higher-lying excited states, thereby suppressing the dielectronic recombination rate. The improved suppression factors presented here, although highly approximate, allow summed recombination rate coefficients to be used to intermediate densities. There have been several technical improvements to our previously presented fits. For H- through B-like ions the activation log densities have been adjusted to better reproduce existing data. For B-, C-, Al-, and Si-like ions secondary autoionization is now included. The treatment of density discontinuity in electron excitations out of ground state H-, He-, and Ne-like ions has been improved. These refined dielectronic recombination suppression factors are used in the most recent version of the plasma simulation code Cloudy. We show how the ionization and emission spectrum change when this physics is included. Although these suppression factors improve the treatment of intermediate densities, they are highly approximate and are not a substitution for a complete collisional-radiative model of the ionization balance.

Variability of the adiabatic parameter in monoatomic thermal and non-thermal plasmas

Context. Numerical models of the evolution of interstellar and integalactic plasmas often assume that the adiabatic parameter γ (the ratio of the specific heats) is constant (5/3 in monoatomic plasmas). However, γ is determined by the total internal energy of the plasma, which depends on the ionic and excitation state of the plasma. Hence, the adiabatic parameter may not be constant across the range of temperatures available in the interstellar medium. Aims. We aim to carry out detailed simulations of the thermal evolution of plasmas with Maxwell–Boltzmann and non-thermal (κ and n) electron distributions in order to determine the temperature variability of the total internal energy and of the adiabatic parameter. Methods. The plasma, composed of H, He, C, N, O, Ne, Mg, Si, S, and Fe atoms and ions, evolves under collisional ionization equilibrium conditions, from an initial temperature of 109 K. The calculations include electron impact ionization, radiative and dielectronic recombinations and line excitation. The ionization structure was calculated solving a system of 112 linear equations using the Gauss elimination method with scaled partial pivoting. Numerical integrations used in the calculation of ionization and excitation rates are carried out using the double-exponential over a semi-finite interval method. In both methods a precision of 10−15 is adopted. Results. The total internal energy of the plasma is mainly dominated by the ionization energy for temperatures lower than 8 × 104 K with the excitation energy having a contribution of less than one percent. In thermal and non-thermal plasmas composed of H, He, and metals, the adiabatic parameter evolution is determined by the H and He ionizations leading to a profile in general having three transitions. However, for κ distributed plasmas these three transitions are not observed for κ &lt; 15 and for κ &lt; 5 there are no transitions. In general, γ varies from 1.01 to 5/3. Lookup tables of the γ parameter are presented as supplementary material.

Electron–ion Recombination Rate Coefficients of Be-like 40Ca16+

Abstract Electron–ion recombination rate coefficients for beryllium-like calcium ions in the center of mass energy from 0 to 51.88 eV have been measured by means of the electron–ion merged-beam technique at the main cooler storage ring at the Institute of Modern Physics in Lanzhou, China. The measurement energy range covers the dielectronic recombination (DR) resonances associated with the core excitations and the trielectronic recombination (TR) resonances associated with the core excitations. In addition, the AUTOSTRUCTURE code was used to calculate the recombination rate coefficients for comparison with the experimental results. Resonant recombination originating from parent ions in the long-lived metastable state ions has been identified in the recombination spectrum below 1.25 eV. A good agreement is achieved between the experimental recombination spectrum and the result of the AUTOSTRUCTURE calculations when fractions of 95% ground-state ions and 5% metastable ions are assumed in the calculation. It is found that the calculated TR resonance positions agree with the experimental peaks, while the resonance strengths are underestimated by the theoretical calculation. Temperature dependent plasma rate coefficients for DR and TR in the temperature range of 103–108 K were derived from the measured electron–ion recombination rate coefficients and compared with the available theoretical results from the literature. In the temperature range of photoionized plasmas, the presently calculated rate coefficients and the recent results of Gu &amp; Colgan et al. are up to 30% lower than the experimentally derived ones, and the older atomic data are even up to 50% lower than the present experimental result. This is because strong resonances situated below electron–ion collision energies of 50 meV were underestimated by the theoretical calculation, which also has a severe influence on the rate coefficients in low-temperature plasmas. In the temperature range of collisionally ionized plasmas, agreement within 25% was found between the experimental result and the present calculation as well as the calculation by Colgan et al. The present result constitutes a set of benchmark data for use in astrophysical modeling.

Influence of the Breit interaction on linear polarization of radiation lines following electron-impact excitation of the boron isoelectronic sequence

Electron-impact excitation cross sections from the ground state $1{s}^{2}2{s}^{2}2{p}_{1/2}$ to the excited states ${[{(1{s}^{2}2s2{p}_{1/2})}_{0}2{p}_{3/2}]}_{3/2}$, ${[{(1{s}^{2}2s2{p}_{3/2})}_{1}2{p}_{3/2}]}_{5/2}$, ${[{(1s2{s}^{2}2{p}_{1/2})}_{0}2{p}_{3/2}]}_{3/2}$, and ${[{(1s2{s}^{2}2{p}_{3/2})}_{1}2{p}_{3/2}]}_{5/2}$ of highly charged boronlike ${\mathrm{Ca}}^{15+}$, ${\mathrm{Xe}}^{49+}$, and ${\mathrm{W}}^{69+}$ ions have been calculated with the fully relativistic distorted-wave method. The degrees of linear polarization of the corresponding radiation lines are further obtained. It is found that the Breit interaction makes the lines corresponding to the $2p\ensuremath{\rightarrow}2s$ transition depolarized, and it makes the ones corresponding to the $2p\ensuremath{\rightarrow}1s$ transition more polarized. These characteristics are very different from the results obtained for the linear polarization of the same radiation lines but formed via the dielectronic recombination process [Phys. Rev. A 91, 042705 (2015)], in which the Breit interaction has no effect on the linear polarization.

Resonant electron scattering and dielectronic recombination in two-center atomic systems

Electron scattering and dielectronic recombination with an ion in the presence of a neighboring atom is studied. The incident electron is assumed to be captured by the ion, leading to resonant excitation of the atom which afterwards may stabilize either by electron reemission or radiative decay. We show that the participation of the atom can strongly affect, both quantitatively and qualitatively, the corresponding processes of electron scattering and recombination. Various ion-atom systems and electronic transitions are considered. In particular we show that electron scattering under backward angles may be strongly enhanced, provided the incident energy lies very close to the resonance. Besides, we derive the scaling behavior of two-center dielectronic recombination with the principal quantum numbers of the participating atomic states. The resonant enhancement in this process is shown to be so strong that it can compete with ordinary single-center recombination even after integrating over a rather broad distribution of incident electron energies.

Triply excited states in dielectronic recombination with excited He-like uranium

We study process of dielectronic recombination with He-like uranium initially being in one of the metastable states $[{(1s2s)}_{0}$ and ${(1s2{p}_{1/2})}_{0}]$. We consider resonant regions of the incident electron energy corresponding to participation of triply excited states of Li-like uranium. The total and differential cross sections for dielectronic recombination as well as the energies and widths of the autoionizing states are calculated accurately within the QED theory. The cross section of the considered process is comparable in order of magnitude with the cross section of dielectronic recombination with H-like or He-like uranium initially being in the ground states. We investigated the contribution of the Breit interaction to the cross section; in particular, we found that the Breit interaction may both significantly increase and decrease the cross section.