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Sökning: WFRF:(Gaigalas Gediminas)

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1.
  • Bengtsson, Peter, et al. (författare)
  • Energy structure and transition rates in the Ne-like sequence from relativistic CI calculations
  • 2012
  • Ingår i: Europhysics Conference Abstracts;36C. - European Physical Society. - 2-914771-75-4 ; :36C
  • Annan publikation (övrigt vetenskapligt)abstract
    • Atomic data are important in astrophysical applications and transition rates can be used in the determination of element abundances and plasma diagnostics. To provide for the extensive data needs a number of general computer codes such as SUPERSTRUCTURE, CIV3, and ATSP2K have been developed. As an alternative to these codes, which all rely on the Breit-Pauli approximation, the fully relativistic GRASP2K code can be used. GRASP2K is based on the multiconfiguration Dirac-Hartree-Fock method and implements a bi-orthogonal transformation method that permits initial and final states in a transition array to be optimized separately, which, in many cases, leads to more accurate values of the resulting rates. The GRASP2K package also contains modules to compute diagonal and off-diagonal hyperfine interaction constants, isotope shifts, Land´e gJ factors, and splittings of magnetic sub-state in intermediate and strong magnetic fields. In this work, GRASP2K has been applied to provide highly accurate spectroscopic data for ions in the Ne-like sequence between Mg III and Kr XXVII. Valence, core-valence, and core-core correlation effects were accounted for through SD-MR expansions to increasing sets of active orbitals. In Mg III, Al IV, Si V, P VI, S VII, and Ar IX, for which experimental energies are known to high accuracy, the mean error in the calculated energies is only 0.011%. For ions with no available experimental energy levels the calculated values should be most valuable in various applications. The high accuracy of the calculated energies makes it possible, in some cases, to to point out experimental values that are in error. Babushkin (length) and Coulomb (velocity) forms of transition rates are computed and agree to within a few percent for the majority of the allowed transitions. Computed lifetimes for states belonging to the 2p33s and 2p53d configurations are in good agreement with values from beam-foil measurements as well as from accurate MCHF Breit-Pauli calculations.
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2.
  • Bieron, Jacek, et al. (författare)
  • Computational Atomic Structure
  • 2012
  • Ingår i: Program and Abstracts : Eighth International Conference on Atomic and Molecular Data and Their Applications: ICAMDATA 8.
  • Annan publikation (övrigt vetenskapligt)abstract
    • There is an increasing demand for accurate atomic data due to advancements in experimental techniques and investments in large scale research facilities. In astrophysics the quality and resolution of solar and stellar spectra has so improved that the accuracy of atomic data is frequently a limiting factor in the interpretation. Accurate atomic data are also required in plasma physics and in other emerging areas such as laser spectroscopy on isotope separators, X-ray lithography, and lighting research. The needs include accurate transition energies, fine- and hyperfine structures, isotope shifts as well as parameters related to interaction with external magnetic fields. Also there is a constant need for transition rates between excited states. Data are needed for a wide range of elements and ionization stages. To meet the demands for accurate atomic data the COMPutational Atomic Structure (COMPAS) group has been formed. The group is involved in developing state of the art computer codes for atomic calculations in the non-relativistic scheme with relativistic corrections in the Breit-Pauli approximation [1] as well as in the fully relativistic domain. Here we describe new developments of the GRASP2K relativistic atomic structure code [2, 3]. We present results for a number of systems and properties to illustrate the potential and restriction of computational atomic structure. Among the properties are hyperfine structures and hyperfine quenched rates, Zeeman splittings in intermediate fields, isotope shifts and transition rates [4]. We also discuss plans for future code developments.
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3.
4.
  • Ekman, Jörgen, et al. (författare)
  • Calculations with spectroscopic accuracy : energies, transition rates, and Landé g_J-factors in the carbon isoelectronic sequence from Ar XIII to Zn XXV
  • 2014
  • Ingår i: Astronomy & Astrophysics. - EDP Sciences. - 0004-6361.
  • Tidskriftsartikel (refereegranskat)abstract
    • Extensive self-consistent multiconfiguration Dirac-Hartree-Fock (MCDHF) calculations and subsequent relativistic configuration in- teraction calculations are performed for 262 states belonging to the 15 configurations 2s22p2, 2s2p3, 2p4, 2s22p3l, 2s2p23l, 2p33l and 2s22p4l (l = 0,1,2) in selected carbon-like ions from Ar XIII to Zn XXV. Electron correlation effects are accounted for through large configuration state function expansions. Calculated energy levels are compared with existing theoretical calculations and data from the Chianti and NIST databases. In addition, Landé gJ -factors and radiative electric dipole transition rates are given for all ions. The accuracy of the calculations are high enough to facilitate the identification of observed spectral lines.
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5.
6.
  • Froese Fischer, Charlotte, et al. (författare)
  • Configuration interaction with separately optimized pair correlation functions
  • 2010
  • Annan publikation (övrigt vetenskapligt)abstract
    • Variational methods produce one-electron radial functions that minimize the total energy of the system. Independent pair correlation functions (PCFs) designed to represent a specific correlation effect – valence, core-valence, or core-core – can be obtained from multiconfiguration Hartree-Fock (MCHF) or Dirac-Hartree-Fock (MCDHF) calculations [1,2]. These separately optimized and nonorthogonal PCFs may then be coupled by solving the associated generalized eigenproblem. In the present study, the Hamiltonian and overlap matrix elements are evaluated through biorthonormal orbital transformations and efficient counter-transformation of the configuration interaction eigenvectors [3]. The ground state of Be atom has been thoroughly tested by this method for various computational strategies and correlation models. It has been shown that the energy convergence is faster than with the usual SD-MCHF method of optimizing a single, orthonormal, one-electron orbital basis spanning the complete configuration space. Beryllium is a small system for which basis saturation can be achieved through complete active space MCHF expansions. But for larger systems describing electron correlation in all space by optimizing a common orthonormal set becomes hopeless whereas the calculation of additional PCFs is straight forward. Our independent optimization scheme, raises many questions related in the choice of the zero-order model to be used when building the interaction matrix. The present study is the first step in the current development of the extension of the atsp2K and grasp2K packages [1,2] that will adopt the biorthonormal treatment for energies, isotope shifts, hyperfine structures and transition probabilities.
7.
  • Froese Fischer, Charlotte, et al. (författare)
  • Doublet-quartet energy separation in boron : a partitioned-correlationfunction- interaction method
  • 2013
  • Ingår i: Physical Review A;6. - American Physical Society. - 1050-2947. ; :6
  • Tidskriftsartikel (refereegranskat)abstract
    • No lines have been observed for transitions between the doublet and quartet levels of B I. Consequently, energy levels based on observation for the latter are obtained through extrapolation of wavelengths along the isoelectronic sequence for the 2s22p 2Po 3/2 – 2s2p2 4P5/2 transition. In this paper, accurate theoretical excitation energies from a partitioned-correlation-function-interaction (PCFI) method are reported for B I that include both relativistic effects in the Breit-Pauli approximation and a finite mass correction. Results are compared with extrapolated values from observed data. For B I our estimate of the excitation energy 28 959 ± 5 cm−1 is in better agreement with the values obtained by Edl´en et al. (1969) than those reported by Kramida and Ryabtsev (2007). Our method is validated by applying the same procedure to the separation of these levels in C II.
8.
  • Gaidamauskas, Erikas, et al. (författare)
  • Tensorial form and matrix elements of the relativistic nuclear recoil operator
  • 2011
  • Ingår i: JOURNAL OF PHYSICS B: ATOMIC, MOLECULAR AND OPTICAL PHYSICS;17. - IOP Publishing Ltd. - 0953-4075. ; :17
  • Tidskriftsartikel (refereegranskat)abstract
    • Within the lowest-order relativistic approximation (∼v2/c2) and to first order in me/M, the tensorial form of the relativistic corrections of the nuclear recoil Hamiltonian is derived, opening interesting perspectives for calculating isotope shifts in the multiconfiguration Dirac–Hartree–Fock framework. Their calculation is illustrated for selected Li-, B- and C-like ions. This work underlines the fact that the relativistic corrections to the nuclear recoil are definitively necessary for obtaining reliable isotope shift values.
9.
  • Gaigalas, Gediminas, et al. (författare)
  • Energies for States of the 2s22p5 and 2s2p6 in Fluorine-like Ions Between Si VI and W LXVI
  • 2012
  • Ingår i: Program and Abstracts : Eighth International Conference on Atomic and Molecular Data and Their Applications: ICAMDATA 8.
  • Annan publikation (övrigt vetenskapligt)abstract
    • The experimental energy levels and computed energies from the largest RCI calculations including QED corrections are displayed in Table 1. The computed energies agree very well with experimental values. Starting from Si VI the energy differences rapidly goes down to a few hundred cm−1 , which corresponds to an error of around 0.02 %. From Sr XXX to Sn XLII experimental energies are given with error bars between 1000 and 2000 cm−1 . The calculated values are within the stated experimental error bars except for Cd XL and Sn XLII. The reason for the difference in these two ions is not known. Experimental data for ions from Sb XLIII to Ta LXV are not available. For the W LXVI ion, the differences between theoretical and experimental transition energies are a few thousand cm−1 . As discussed by Kramida [1] the total uncertainties of the measured energies in W LXVI were dominated by the calibration uncertainties and varied in the range 1.0 - 2.3 eV, which translates to 8000 - 20000 cm−1 . Based on the comparison between theory and experiment for the lighter ions as well as for W LXVI we estimate that the errors in the calculated transition energies for ions in the range Sb XLIII - Ta LXV, for which no experimental data are available, are less than 0.08 %.
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10.
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  • Resultat 1-10 av 63
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