Search Results (170 total)

Calculations using multiconfiguration Breit-Pauli and multiconfiguration Dirac-Fock methodologies have revealed that the radiative and Auger rates, and the associated fluorescence yields, of the six electron 1s2s²2p³ K-shell vacancy isoelectronic sequence exhibit nonmonotonic behavior as a function of nuclear charge Z. This behavior is explained in terms of an accidental degeneracy, an avoided crossing of two nearly degenerate spin-orbit coupled levels. The results also demonstrate the importance of including both electron-electron correlation and spin-orbit effects even at low Z.

The hyperfine-induced 2s2p  ³P₀–2s^{2  1}S₀ transition rate for Be-like ⁴⁷Ti¹⁸⁺ was recently measured in a storage-ring experiment by Schippers [Phys. Rev. Lett. 98, 033001 (2007)]. The measured value of 0.56(3)  s⁻¹ is almost 60% larger than the theoretical value of 0.356  s⁻¹ from a multiconfiguration Dirac-Fock calculation by Marques [Phys. Rev. A 47, 929 (1993)]. In this work, we use a large-scale relativistic configuration-interaction method to calculate these hyperfine-induced rates for ions with Z=6–92. Coherent hyperfine-quenching effects between the 2s2p  ^1,3P₁ states are included in a perturbative as well as a radiation damping approach. Contrary to the claims of Marques , contributions from the ¹P₁ state are substantial and lead to a hyperfine-induced rate of 0.67  s⁻¹, in better agreement with, though larger than, the measured value.

By the spatial-temporal-resolved femtosecond spectroscopy on well-textured (110)- and (100)-YBCO thin films, two distinctly temperature-dependent characteristics of quasiparticle relaxation in the nodal and antinodal directions are clearly identified. One temperature dependence associated with a high-energy gap has been observed along the ab diagonal for the whole hole-doping region and along the b axis except near optimal doping, while the other temperature dependence related to the opening of the superconducting gap appears along b axis regardless of the hole concentration. This spatial dichotomy between the nodal and antinodal quasiparticle dynamics and the evolution of gap symmetry with hole doping are discussed for enlightenment on the nature of the phase diagram of hole-doped cuprates. These strongly suggest that the two characteristics may have different physical origins and compete with each other.

Quantum electrodynamic (QED) corrections to 4p-4d transition energies of several copperlike ions with Z=70–92 are calculated nonperturbatively in strong external fields to all orders in binding corrections. Dirac-Kohn-Sham potentials are used to account for screening and core-relaxation effects. For the 4p_1∕2-4d_3∕2 transition in copperlike bismuth, thorium, and uranium, results are in good agreement with empirical QED corrections deduced from differences between transition energies obtained from recent high-precision electron-beam ion-trap measurements and those calculated with the relativistic many-body perturbation theory (RMBPT). These comparisons provide sensitive tests of QED corrections for high-angular-momentum states in many-electron heavy ions and illustrate the importance of core-relaxation corrections. Comparisons are also made with other theories and with experiments on the 4s-4p transition energies of high-Z Cu-like ions as accuracy checks of the present RMBPT and QED calculations.

The spatially resolved relaxation characteristics of photoinduced quasiparticles (QPs) in CuO₂ planes of underdoped YBCO are disclosed by polarized fs time-resolved spectroscopy. The relaxation time (τ) along b axis diverges at T_c, and appears to be governed by a temperature-dependent gap Δ(T) at T<T_c. Furthermore, for T>T_c, a monotonic increase of τ with decreasing T along the b axis and ab diagonal was observed and can be attributed to a temperature-independent gap Δ_p. The results lend support to the recombination dominant scenario of QP dynamics. However, the QP thermalization may take part along the nodal direction in the highly underdoped samples.

The polarized femtosecond spectroscopies obtained from well characterized (100) and (110) YBa₂Cu₃O_7-δ thin films are reported. This bulk-sensitive spectroscopy, combined with the well-textured samples, serves as an effective probe to quasiparticle relaxation dynamics in different crystalline orientations. The significant anisotropy in both the magnitude of the photoinduced transient reflectivity change and the characteristic relaxation time indicates that the nature of the relaxation channel is intrinsically different in various axes and planes. By an orientation-dependent analysis, d-wave symmetry of the bulk-superconducting gap in cuprate superconductors emerges naturally.

Spectral lines from the 3s-3p_3/2 transitions in Na-like to P-like uranium have been measured at the Lawrence Livermore National Laboratory EBIT-I electron-beam ion trap facility. The measured x-ray energies are among the most accurate spectroscopic data available for highly charged heavy ions and provide important tests of relativistic atomic structure theory as well as bound-state quantum electrodynamic (QED) corrections in strong fields. Relativistic configuration-interaction (RCI) calculations based on the no-pair Hamiltonian with B-spline basis functions have been carried out to study these transitions. Our RCI energies, corrected by many-electron screened QED corrections also calculated here, yield theoretical results in very good agreement with experiment.

Double K-shell photoionization of Ne at 5000 eV was observed by recording the KK-KLL Auger-electron hypersatellite spectrum. The measured Auger spectrum is compared with the results of multiconfiguration Dirac-Fock calculations. Shake calculations are used to identify likely multivacancy states produced by photoexcitation or ionization in addition to double-K vacancies, and their calculated Auger spectra are compared with the measured spectrum. The measured relative intensities of hypersatellite and diagram Auger lines are combined with experimental and theoretical determinations of branching ratios from single- and double-K vacancies into final states to determine the ratio of double-to-single K-shell photoionization cross sections to be 0.32(4)%. This ratio is much larger than the calculated high-energy-limit ratio and indicates a large contribution of dynamic electron correlation.

The wavelength of the 3s_1/2-3p_3/2 transition of Na-like U⁸¹⁺ was determined to be 9.499 85±0.000 15 Å (1305.12±0.02 eV), using the EBIT-I electron beam ion trap. The measurement is many times more sensitive to the radiative contributions from quantum electrodynamics than earlier measurements for Pt⁶⁷⁺ and Pb⁷¹⁺. Our result strongly deviates from various predictions employing scaled hydrogenic quantum electrodynamic corrections and establishes a benchmark for multielectron QED calculations that agrees well with the trend established by ab initio calculations.

In the modeling of the astrophysical plasmas, the relative elemental abundance inferred from solar and stellar upper atmosphere can be affected by as much as a factor of 5 due to the uncertainties in the current dielectronic recombination (DR) rate coefficients used to analyze the spectra [Savin and Laming, Astrophys. J. 566, 1166 (2002)]. DR rate coefficients for oxygenlike ions have been identified as the most urgent needs for the astrophysical applications. In this work, we report on the calculations of DR rate coefficients for Mg V, Si VII, S IX, and Fe XIX ions which are important for the modeling of the astrophysical plasmas. The calculations are carried out in isolated resonance and distorted-wave approximations. The relevant atomic data are calculated using the multiconfigurational Dirac-Fock method. We include 2s-2p, 2p_1/2-2p_3/2, 2l-3l^′, and 1s-2p excitations and cover temperatures ranging from 0.001 eV to 10 000 eV. For low temperatures, it is essential to have accurate DR resonance energies and to include fine-structure excitations in order to obtain reliable DR rate coefficients. Good agreement with experiment has been found for Fe XIX. For Mg V, Si VII, and S IX, significant discrepancies are noted between this work and recommended rate coefficients.

The computation of superconfiguration partition functions relies upon independent electron statistics, with electron-electron contributions included as an average first-order correction factor. The decomposition into a first-order correction and reference independent electron system has degrees of freedom not exploited by current methods. We present a derivation for the conventional choice of decomposition and propose a different method for obtaining an optimal decomposition for each superconfiguration. This constitutes an alternative procedure to recomputing self-consistent fields for the refinement of superconfiguration partition functions. Numerical results are presented and discussed.

A large-scale, relativistic configuration-interaction (RCI) method has been developed for precision calculations of transition oscillator strengths. It is based on the no-pair Hamiltonian and employs finite B-spline basis functions. For the 2s^{2 1}S₀–2s2p ³P₁ intercombination transition in berylliumlike carbon, the present RCI expansions reach close to 200 000 configurations, and include all single and double excitations from valence-valence, core-valence, and core-core interactions, along with dominant triple and quadruple excitations. Resulting length- and velocity-gauge transition rates are very well converged, but still differ by a factor of 2. This strong gauge dependence is found to arise from the neglect of negative-energy states which has negligible effects on length-gauge results but can affect velocity-gauge results significantly. The present intercombination transition rate for C III of 101.6±0.7 sec^-1 differs from the measured value of 102.94±0.14 sec^-1 [Doerfert et al., Phys. Rev. Lett. 78, 4355 (1997)] by about 1.3%.

The microscopic spatiotemporal response of a quasi-two-dimensional dust Coulomb liquid to the shear induced by a cw laser is investigated through optical microscopy. The dust Coulomb liquid consists of many micrometer sized dust particles charged and suspended in a low pressure rf discharge background. Assisted by thermal fluctuations, the laser forcing enhances the cascaded generation of irregular vortices through reducing caging barriers for collective hopping. The vortex mixing leads to the mean velocity field with a simple structure that has a strong shear along the edge of the narrow laser beam. It also promotes the anomalous transverse diffusion with decaying strength from the line source. The viscosity and diffusion coefficient both show nonlinear dependence on the laser power under the interplay among the above nonlinear excitation and relaxation processes.

Relativistic configuration-interaction (RCI) calculations of the correlation energies of the 2s-2p_3/2 and the 2s^{2 1}S₀-2s2p ¹P₁ transitions in high-Z Li-like and Be-like ions, respectively, are reexamined. Experimental quantum electrodynamic (QED) corrections are obtained by subtracting RCI correlation energies from high-precision x-ray energy measurements. It is found that our QED results are in good agreement with experiment and that screening and relaxation corrections are well accounted for by the use of the Dirac-Kohn-Sham potentials in QED calculations of the one-loop radiative diagrams. Our present QED energies are also in good agreement with recent higher-order QED calculations of the 2s-2p_1/2 transition in high-Z Li-like ions.

We have measured resonant strengths for Δn>~1 dielectronic recombinaton in heliumlike Ar¹⁶⁺. The measurements were performed with an electron beam ion trap and include the KLM through KLP resonances contributing to n→1 K-shell x-ray emission with n=2-6. The measured values agree within a few percent with relativistic multiconfiguration calculations.

We have measured the ratio of the intensity of the n=3→1 intercombination line (1s3p ³P₁→1s^{2 1}S₀) to that of the resonance line (1s3p ¹P₁→1s^{2 1}S₀) for the heliumlike ions Mg¹⁰⁺, Al¹¹⁺, Ar¹⁶⁺, Ti²⁰⁺, V²¹⁺, Cr²²⁺, and Fe²⁴⁺. The measurements were carried out on the Lawrence Livermore National Laboratory electron-beam ion trap using electron-beam energies mostly near the threshold for the direct excitation of the resonance line, or near the threshold for excitation of the n=4 levels, to avoid populating the n=3 levels by cascades from levels with n>~5. High-resolution Bragg crystal spectrometers in curved as well as flat geometry were used to record the x rays. The measured ratios have been compared to predicted values, which were found to underestimate the measured values.

Energy levels of the ground state and n=2 excited states of heliumlike ions with 22<~Z<~36 are calculated using a large-scale, relativistic configuration-interaction method. Quantum electrodynamic corrections are evaluated in Dirac-Kohn-Sham (DKS) potentials to account for screening and relaxation effects, and results are shown to be quite reliable as long as the Latter correction to the DKS potentials is excluded. We also find good agreements among different high-precision calculations and between theory and experiment in this Z range.

Solving the relativistic many-body problem using either many-body perturbation theory or configuration-interaction techniques is shown to lead to energies that depend slightly on the starting potential if the effects of virtual electron-positron pairs are excluded. If they are included without a correct implementation of quantum electrodynamics (QED), while this dependence is eliminated in a mathematical sense, the procedure is shown to give unphysical answers. When instead the effects of virtual electron-positron pairs are included using the S-matrix theory implementation of QED, the potential dependence is shown to be eliminated in a physical way through the inclusion of a particular class of Feynman diagrams.

We have calculated the total dielectronic recombination (DR) coefficients for the ²P_1/2 and ²P_3/2 states in B-like Ti¹⁷⁺, Fe²¹⁺, and Mo³⁷⁺ ions for electron temperatures 0.1<~T<~10 000 eV. The calculations are carried out using the multiconfiguration Dirac-Fock method in intermediate coupling with a configuration interaction. We find that accurate Coster-Kronig energies are critical for a successful determination of low-temperature DR coefficients. We also find that the DR involving fine-structure excitations can be as important as the 2s-2p excitation channels in the low-temperature regime for some ions.

Based on precise spectroscopic measurements of the x-ray transitions of few-electron uranium ions, the difference in the nuclear charge radii of ²³⁵U and ²³⁸U has been determined. The measurement makes use of the simplified atomic structure of highly charged ions, and it is shown that the spectroscopic data from few-electron ions are readily interpreted in terms of the variation in the mean nuclear radius. A value of δ〈r²〉^235,238=-0.250±0.032 fm² is found. Combining the results with data from earlier optical and x-ray measurements, values for δ〈r²〉^A,238 with A=233, 234, 235, and 236 are derived that are of higher accuracy than previous values.

We have calculated the 3s²¹S₀-3s3p^1,3P₁ transition energies for neutral magnesium and Mg-like Ar⁶⁺, Cu¹⁷⁺, Kr²⁴⁺, and Mo³⁰⁺ using the relativistic configuration-interaction (CI) method. These calculations are based on the relativistic no-pair Hamiltonian which includes Coulomb and retarded Breit interactions and employ finite B-spline basis functions. Quantum electrodynamic and mass polarization corrections are also calculated. For Mg-like ions studied here, intravalence correlations are treated exactly by saturating the basis with all configurations that arise from valence-valence excitations. Contributions from core polarization are also calculated with large-scale CI expansions by including dominant configurations from core-valence excitations. Agreement between theory and experiment are good for all the Mg-like ions studied here.