Resonant Recombination Research Articles

Breit interaction in dielectronic recombination of hydrogenlike xenon ions: storage-ring experiment and theory

Abstract Electron-ion collision spectroscopy of the KLL dielectronic recombination (DR) resonances of hydrogenlike xenon ions was performed at a heavy-ion storage ring with a resolving power that is competitive with x-ray spectroscopy of inner-shell transitions in highly charged ions. The $$KL_{1/2}L_{1/2}$$ K L 1 / 2 L 1 / 2 , $$KL_{1/2}L_{3/2}$$ K L 1 / 2 L 3 / 2 , and $$KL_{3/2}L_{3/2}$$ K L 3 / 2 L 3 / 2 resonance groups and even parts of their fine structure are individually resolved. The resonance strengths were measured on an absolute scale and compared with results from multi-configuration Dirac–Fock (MCDF) calculations. These are in excellent agreement with the experimental findings when QED effects on the resonance energies and the Breit interaction are considered. As already found for DR of hydrogenlike uranium (Bernhardt et al. in Phys Rev A 83:020701(R), 2011), this interaction is particularly strong for the $$KL_{1/2}L_{1/2}$$ K L 1 / 2 L 1 / 2 resonance group. For U$$^{91+}$$ 91 + , it increases the $$KL_{1/2}L_{1/2}$$ K L 1 / 2 L 1 / 2 DR resonance strength by 40%. For Xe$$^{53+}$$ 53 + , the increase is found to amount to 25%, confirming the prediction that the influence of the Breit interaction grows with increasing nuclear charge. A comprehensive appendix treats the derivation of experimental and theoretical merged-beams recombination rate coefficients for interacting beams of relativistic electrons and ions. Graphical abstract

Effect of Nonequilibrium Electron-Energy Distributions on Efficiency of Rydberg State Population via Resonant Recombination

Effect of Nonequilibrium Electron-Energy Distributions on Efficiency of Rydberg State Population via Resonant Recombination

Resonant defect recombination-localized surface plasmon energy transfer and exciton dominated fluorescence in ZnO-Au-ZnO multi-interfaced heteronanocrystals.

The semiconductor-metal heteronanocrystals (HNCs) that possess a perfect epitaxial interface can accommodate novel and interesting physical phenomena owing to the strong interaction and coupling between the semiconductor excitons and metal plasmons at the interface. Here, we fabricate the pyramidal ZnO-Au HNCs and study their unique photophysical properties. Several Au nanospheres are perfectly epitaxially bound with a single ZnO NC owing to the small lattice mismatch between them and there are also ZnO-Au-ZnO sandwiched HNCs. There is a strong coupling between the green defect-associated recombination in the ZnO NC and the localized surface plasmon resonance (LSPR) of the Au nanosphere at the interface of the HNC. This leads to resonant defect recombination-LSPR energy transfer and resultant nearly complete quenching of the green defect luminescence of the ZnO NCs in the HNCs, leaving only the UV exciton luminescence. The lifetimes of both the green and UV emission bands decrease significantly in the ZnO-Au HNCs relative to that of the pure ZnO NCs owing to the combined effect of resonance energy transfer and surface plasmon enhanced radiative transition. The exponent of the luminescence intensity-excitation intensity power function for the green emission band is remarkably smaller than unity, and this suggests that the involved defects have an intermediate concentration.

Collectivity of J/ψ Mesons in Heavy-Ion Collisions.

The production of J/ψ mesons in heavy-ion collisions at the Large Hadron Collider is believed to be dominated by the recombination of charm and anticharm quarks in a hot QCD medium. However, measurements of the elliptic flow (v_{2}) of J/ψ mesons in these reactions are not well described by existing calculations of J/ψ recombination for transverse momenta p_{T}≳4 GeV. We revisit these calculations in two main aspects. Employing the resonance recombination model, we implement distribution functions of charm quarks transported through the quark-gluon plasma using state-of-the-art Langevin simulations and account for the space-momentum correlations of the diffusing charm and anticharm quarks in a hydrodynamically expanding fireball. This extends the relevance of the recombination processes to substantially larger momenta than before. We also revisit the suppression of primordially produced J/ψ's by propagating them through the same hydrodynamic medium, leading to a marked increase of their v_{2} over previous estimates. Combining these developments into a calculation of the p_{T}-dependent nuclear modification factor and v_{2} of inclusive J/ψ production in semicentral Pb-Pb collisions at the LHC, we find a good description of the experimental results by the ALICE Collaboration. Our results thus resolve the abovementioned v_{2} puzzle and imply the relevance of recombination processes for p_{T}'s of up to ∼8 GeV.

Calculations of electron-impact excitation and dielectronic recombination rate coefficients of highly charged silicon ions

The rate coefficients for dielectronic recombination and electron-impact excitation processes with H-like to Be-like silicon ions (${\mathrm{Si}}^{13+}\text{\ensuremath{-}}{\mathrm{Si}}^{10+}$) are calculated using relativistic distorted-wave approach. The prominent $▵n=1$ and the weaker $▵n=2$ and 3 dielectronic recombination (DR) resonances with $K$-shell excitations are presented, and compared with the existing DR experimental rate coefficients of ${\mathrm{Si}}^{13+}, {\mathrm{Si}}^{12+}$, and ${\mathrm{Si}}^{11+}$ ions, it is found that the theoretical DR rate coefficients are in very good agreement with the experimental results. With the same approach, the direct and resonant electron-impact excitation (EIE) cross sections associated with $1s\ensuremath{-}{n}^{\ensuremath{'}}{l}^{\ensuremath{'}}$ core excitations are calculated for the ground states of ${\mathrm{Si}}^{13+}\text{\ensuremath{-}}{\mathrm{Si}}^{10+}$ ions and found in excellent agreement with the available experiment. Finally, we present the synthesized DR and EIE rate coefficients for the sum of all detected $(13+\ensuremath{\sim}10+)$ charge states of Si and these agree well with the experimental results.

Synergistic effects of Ag inclusion in titania on its photocatalytic activity for the removal of methyl orange

Uniformly distributed Ag nanoparticles are successfully loaded on TiO2 nanoparticles with different Ag concentration through hydrothermal method. X-ray diffraction pattern and Raman spectra demonstrate the anatase phase of both TiO2 and Ag/TiO2. Loading of Ag on the surface of TiO2 shifts the wavelength of absorption edge to visible light resulting the narrow energy band gap. The photoluminescence intensity decreases with an increase in the concentration of Ag revealing a decrease in the recombination of electron-hole pairs. In order to test them in the photocatalysis, the degradation of textile dye methyl orange is used as a standard. Ag loaded TiO2 shows better photocatalytic performance than pure TiO2 because the higher surface area of the nanoparticles enabled them to function as good electron acceptors, delayed the electron-hole pair recombination and surface plasmon resonance of the silver particles. Moreover, 0.1 M of Ag/TiO2 nanoparticles (AT3) showed higher photocatalytic efficiency of 98.40% within 240 min light irradiation. The superior photocatalytic activity of Ag/TiO2 is attributed to the interactive effect caused by smaller crystallite size, narrow band gap, large surface area, and also enhancement in the charge separation.

Tailoring Nanostructures of Quantum Dots toward Efficient and Stable All-Solution Processed Quantum Dot Light-Emitting Diodes.

Quantum dots (QDs) light-emitting diodes (QLEDs) are considered the most promising candidate for application in displays. While the efficiency of QLEDs has been greatly developed in recent years and is comparable to that of organic light-emitting diodes (OLEDs), it still remains challenging to realize both high efficiency and long lifetimes. In this work, we report efficient and stable red QLEDs with the maximum current efficiency of 13.48 cd A-1, external quantum efficiency of 18.65%, and low efficiency roll-off at high luminance with a long lifetime exceeding ∼2.9 × 105 h, representing a 3-fold increase in stability. Tailoring the composition of QDs suppresses nonradiative Förster resonant energy transfer and Auger recombination and provides favorable valence band alignment to boost the hole injection. Our work suggests that tailoring the nanostructures of QDs offers an effective means to simultaneously achieve high efficiency and high stability, accelerating QLED technology for practical applications in displays and lighting.

Charm-hadron production in pp and AA collisions

Recent measurements of various charm-hadron ratios in pp, p-Pb and Pb-Pb collisions at the LHC have posed challenges to the theoretical understanding of heavy-quark hadronization. The Λc/D0 ratio in pp and p-Pb collisions shows larger values than those found in e+e− and ep collisions and predicted by Monte-Carlo event generators based on string fragmentation, at both low and intermediate transverse momenta (pT). In AA collisions, the Ds/D ratio is significantly enhanced over its values in pp, while the Λc/D0 data indicates a further enhancement at intermediate pT. Here, we report on our recent developments for a comprehensive description of the charm hadrochemistry and transport in pp and AA collisions. For pp collisions we find that the discrepancy between the Λc/D0 data and model predictions is much reduced by using a statistical hadronization model augmented by a large set of “missing” states in the charm-baryon spectrum, contributing to the Λc via decay feeddown. For AA collisions, we develop a 4-momentum conserving resonance recombination model for charm-baryon formation implemented via event-by-event simulations that account for space-momentum correlations (SMCs) in transported charm- and thermal light-quark distributions. The SMCs, together with the augmented charm-baryon states, are found to play an important role in describing the baryon-to-meson enhancement at intermediate momenta. We emphasize the importance of satisfying the correct (relative) chemical equilibrium limit when computing the charm hadrochemistry and its momentum dependence with coalescence models.

Electron-ion recombination rate coefficients of carbon-like Ar12+

Electron-ion recombination of carbon-like Ar12+ forming Ar11+ has been investigated for the first time by using the cooler storage ring CSRm at the Institute of Modern Physics in Lanzhou, China. The absolute recombination rate coefficients are derived from the measurement in the electron-ion collision energy range of 0–50 eV, covering dielectronic recombination (DR) resonances associated with 2s22p2 and 2s2p3 (Δn = 0) core excitations. Theoretical results are obtained by employing FAC code and compared with the experimental recombination spectrum. An overall agreement is found in resonance energy positions between merged-beam and calculated spectra. Temperature dependent rate coefficients are derived from the measured DR spectrum by convoluting it with a Maxwell–Boltzmann energy distribution and compared with the calculations from the literature. In the presented temperature range 103–107 K, the experimentally derived rate coefficients agree with the theoretical results of Gu (2003 Astrophys. J. 590 1131) within the experimental uncertainties. The calculation by Zatsarinny et al (2004 Astron. Astrophys. 417 1173) underestimates the rate coefficients in the temperature range 103–5 × 104 K. The combination of the experimental results and theoretical calculation provides a benckmark for Ar12+ recombination data used in astrophysical modeling.

Photoluminescent Spectral Broadening of Lead Halide Perovskite Nanocrystals Investigated by Emission Wavelength Dependent Lifetime.

Despite intensive efforts, the fluorescence of perovskite nanocrystals (NCs) still suffers from a poor color purity, which limits the applications in light emitting and multicolor display. A deep understanding on the fundamental of the photoluminescent (PL) spectral broadening is thus of great significance. Herein, the PL decay curves of the CsPbClxBr3-x NCs are monitored at different wavelengths covering the entire PL band. Moreover, energy relaxation time τ and radiative recombination time β are obtained by numerical fittings. The dependences of τ and 1/β on the detection wavelength agree well with the steady-state PL spectrum, indicating the observed PL broadening is an intrinsic effect due to the resonance and off-resonance exciton radiative recombination processes. This work not only provides a new analysis method for time-resolved PL spectra of perovskites, but also gains a deep insight into the spectral broadening of the lead halide perovskite NCs.

Rydberg states population via three-body and dissociative recombination in low-temperature plasmas of rare gas mixtures

We study various physical mechanisms of electron-ion recombination accompanied by the formation of Rydberg atoms in plasmas containing atomic and molecular ions. Analytical approach to the description of resonant mechanism of ternary electron capture associated with the non-adiabatic energy transfer from free electron to the electronic shell of a quasimolecular ion is developed. Similar technique is used for the evaluation of the integral contribution of all rovibrational states of a bound molecular ion to the Boltzmann-averaged cross section and rate constant of dissociative recombination (DR). Simple expressions for cross sections and rate constants of non-resonant electron capture to Rydberg states in ternary electron-ion-atom collisions are derived in the impulse approximation. We perform a comparative analysis of the efficiencies of resonant and non-resonant three-body recombination of electrons with atomic ions and DR of molecular ions in plasmas of rare gas mixtures, Rg/Xe, with small relative concentration of xenon ([Xe] ≪ [Rg], Rg = He, Ne, Ar, Kr). It is shown that in a wide range of plasma parameters the resonant recombination mechanisms are predominant. The behavior and magnitudes of the recombination rate constants turn out to be quite different for heteronuclear ions with small (HeXe+ and NeXe+, D0 = 13.1 and 33 meV) and moderate (ArXe+ and KrXe+, D0 = 171 and 400 meV) values of the dissociation energy.

Quantum‐Dot Light‐Emitting Diodes for Outdoor Displays with High Stability at High Brightness

AbstractQuantum dot light‐emitting diodes (QLEDs) are considered to be the candidate light sources with the most potential for applications in displays. Recent advances in luminance, external quantum efficiency (EQE), and even the operation lifetime of QLEDs have already satisfied the requirements for low‐light‐level displays. However, the short operation lifetime under high brightness limits the application of QLEDs for outdoor displays and lightings. Here, demonstrated are green QLEDs with a T95 operation lifetime reaching up to 2500 h at high brightness (1000 cd m−2) with a high peak EQE of 23.9%, current efficiency of 100.5 cd A−1, and low efficiency roll‐off at high current. Both the EQE and lifetime of the QLEDs are superior to those reported to date for all solution‐processed green QLEDs. These major advances are qualitatively attributed to the use of a shell‐tailoring strategy for producing compositional graded CdZnSe/ZnSe/ZnSeS/ZnS quantum dots with a high photoluminescence quantum yield, suppressed nonradiative Förster resonant energy transfer and Auger recombination, and favorable valence band alignment for enhanced hole injection. Collectively, this work represents a huge step forward in eventually realizing QLEDs for high‐brightness display and lighting applications.

Dielectronic recombination rate coefficients of fluorine-like nickel

Electron-ion recombination rate coefficients for fluorine-like nickel ions have been measured by employing the merged-beam technique at the cooler storage ring CSRm at the Institute of Modern Physics in Lanzhou, China. The measured spectrum covers the energy range of 0–160 eV, including all the dielectronic recombination (DR) resonances associated with ΔN = 0 core excitations. The DR cross sections in this energy range were calculated by a relativistic configuration interaction method using the flexible atomic code (FAC). Radiative recombination (RR) cross sections were obtained from a modified version of the semi-classical Bethe & Salpeter (1957, Quantum Mechanics of One- and Two-Electron 56 Systems (Springer)) formula for hydrogenic ions. The comparison between the measurement and the calculation shows that the present theoretical model still needs to be improved at low collision energies. Temperature dependent plasma recombination rate coefficients were derived from the measured DR rate coefficients in the temperature range of 103–108 K and compared with the presently calculated result as well as previous available data in the literature. The experimentally derived data agree well with the theoretical calculations for temperatures where Ni19+ ions form in collisionally ionized plasmas. At lower temperatures typical for photo-ionized plasmas, discrepancies are found beyond the experimental uncertainty, which can be attributed to the disagreement between the measurement and the calculation of the low-lying DR resonances. The present experimental result benchmarks the plasma DR rate coefficients, in particular for temperatures below 105 K where the ΔN = 0 DR resonances dominate.

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.

Pd/TiO2 Nanospheres with Three-dimensional Hyperstructure for Enhanced Photodegradation of Organic Dye

Pd/TiO2 nanospheres assembled from nanorods with three-dimensional hyperstructure for enhanced photodegradation of organic dye was prepared by a sample solvothermal method. Further, it was used for degradation of methyl orange(MO) under the illumination of visible-light(simulated). The results show that Pd/TiO2 with 2%(mass fraction) Pd exhibits the noticeable activity in photodegrading of MO. The detailed analysis shows that this enhancement is attributed to the reduction of the rate of electron-hole recombination and surface plasmon resonance of Pd. The high activity and good stability of the obtained Pd/TiO2 nanospheres make it a promising photocatalyst for solving the environmental pollution problems.

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.

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 & 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.

K-shell excitation dielectronic recombination resonance strengths of highly charged He-like to O-like Xe ions

Dielectronic recombination is an important process in high temperature plasmas. In the present work, the KLn (n = L, M, N and O) DR resonance strengths of He-like to O-like xenon ions are measured at the Shanghai electron beam ion trap using a fast electron beam energy scanning method. The experiment uncertainty reaches about 6% with significant improvement of statistics. A relativistic configuration interaction calculation is also made. Theoretical results agree with the experiment results within 15% in most cases.

Dielectronic and Trielectronic Recombination Rate Coefficients of Be-like Ar 14+

Electron–ion recombination of Be-like 40Ar14+ has been measured by employing the electron–ion merged-beams method at the cooler storage ring CSRm. The measured absolute recombination rate coefficients for collision energies from 0 to 60 eV are presented, covering all dielectronic recombination (DR) resonances associated with 2s 2 → 2s2p core transitions. In addition, strong trielectronic recombination (TR) resonances associated with 2s 2 → 2p 2 core transitions were observed. Both DR and TR processes lead to series of peaks in the measured recombination spectrum, which have been identified by the Rydberg formula. Theoretical calculations of recombination rate coefficients were performed using the state-of-the-art multi-configuration Breit–Pauli atomic structure code AUTOSTRUCTURE to compare with the experimental results. The plasma rate coefficients for DR+TR of Ar14+ were deduced from the measured electron–ion recombination rate coefficients in the temperature range from 103 to 107 K, and compared with calculated data from the literature. The experimentally derived plasma rate coefficients are 60% larger and 30% lower than the previously recommended atomic data for the temperature ranges of photoionized plasmas and collisionally ionized plasmas, respectively. However, good agreement was found between experimental results and the calculations by Gu and Colgan et al. The plasma rate coefficients deduced from experiment and calculated by the current AUTOSTRUCTURE code show agreement that is better than 30% from 104 to 107 K. The present results constitute a set of benchmark data for use in astrophysical modeling.