696 research outputs found
Relativistic fine structure oscillator strengths for Li-like ions: C IV - Si XII, S XIV, Ar XVI, Ca XVIII, Ti XX, Cr XXII, and Ni XXVI
Ab initio calculations including relativistic effects employing the
Breit-Pauli R-matrix (BPRM) method are reported for fine structure energy
levels and oscillator strengths upto n = 10 and 0.leq. l .leq.9 for 15 Li-like
ions: C IV, N V, O VI, F VII, Ne VIII, Na IX, Mg X, Al XI, Si XII, S XIV, Ar
XVI, Ca XIII, Ti XX, Cr XXII, and Ni XXVI. About one hundred bound fine
structure energy levels of total angular momenta, 1/2 .leq. J .leq. 17/2 of
even and odd parities, total orbital angular momentum, 0 .leq L .leq. 9 and
spin multiplicity (2S+1) = 2, 4 are considered for each ion. The levels provide
almost 900 dipole allowed and intercombination bound-bound transitions. The
BPRM method enables consideration of large set of transitions with uniform
accuracy compared to the best available theoretical methods. The CC
eigenfunction expansion for each ion includes the lowest 17 fine structure
energy levels of the core configurations 1s^2, 1s2s, 1s2p, 1s3s, 1s3p, and
1s3d. The calculated energies of the ions agree with the measured values to
within 1% for most levels. The transition probabilities show good agreement
with the best available calculated values. The results provide the largest sets
of energy levels and transition rates for the ions and are expected to be
useful in the analysis of X-ray and EUV spectra from astrophysical sources.Comment: 16 pgs., to appear in Astronomy and Astrophysic
Recombination Rate Coefficients for KLL Di-electronic Satellite Lines of Fe XXV and Ni XXVII
The unified method for total electron-ion recombination is extended to study
the dielectronic satellite (DES) lines. These lines, formed from radiative
decay of autoionizing states, are highly sensitive temperature diagnostics of
astrophysical and laboratory plasma sources. The computation of the unified
recombination rates is based on the relativistic Breit-Pauli R-matrix method
and close coupling approximation. Extending the theoretical formulation
developed earlier we present recombination rate coefficients for the 22
satellite lines of KLL complexes of helium-like Fe XXV and Ni XXVII. The
isolated resonance approximation, commonly used throughout plasma modeling,
treats these resonances essentially as bound features except for dielectronic
capture into, and autoionization out of, these levels. A line profile or cross
section shape is often assumed. On the other hand, by including the coupling
between the autoionizing and continuum channels, the unified method gives the
intrinsic spectrum of DES lines which includes not only the energies and
strengths, but also the natural line or cross section shapes. A formulation is
presented to derive autoionization rates from unified resonance strengths and
enable correspondence with the isolated resonance approximation. While the
rates compare very well with existing rates for the strong lines to <20%, the
differences for weaker DES lines are larger. We also illustrate the application
of the present results to the analysis of K ALPHA complexes observed in
high-temperature X-ray emission spectra of Fe XXV and Ni XXVII. There are
considerable differences with previous results in the total KLL intensity for
Fe XXV at temperatures below the temperature of maximum abundance in coronal
equilibrium. (Abbreviated Abstract)Comment: 21 pages, 5 figures, to appear in Physica Script
Electron impact ionization of metastable 2P-state hydrogen atoms in the coplanar geometry
AbstractTriple differential cross sections (TDCS) for the ionization of metastable 2P-state hydrogen atoms by electrons are calculated for various kinematic conditions in the asymmetric coplanar geometry. In this calculation, the final state is described by a multiple-scattering theory for ionization of hydrogen atoms by electrons. Results show qualitative agreement with the available experimental data and those of other theoretical computational results for ionization of hydrogen atoms from ground state, and our first Born results. There is no available other theoretical results and experimental data for ionization of hydrogen atoms from the 2P state. The present study offers a wide scope for the experimental study for ionization of hydrogen atoms from the metastable 2P state
Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. IX. Ni XXVI and Ni XXVII for UV and X-ray modeling
The inverse processes of photoionization and electron-ion recombination of
(hnu + Ni XXVI --> Ni XXVII + e and (hnu + Ni XXVII --> Ni XXVIII + e) are
studied using the unified method for the total recombination. The method
subsumes both the radiative and di-electronic recombination processes and
enables self-consistent sets of results for photoionization and electron-ion
recombination by using the same wavefunction expansion. Photoionization cross
sections (sigma_PI), recombination cross sections (sigma_{RC}), recombination
collision strengths (Omega_{RC}), and recombination rate coefficients
(alpha_{RC}) are obtained for ionization balance and spectral analysis of UV
and X-ray lines. Level-specific photoionization cross sections and
recombination rates are presented to enable accurate computation of
recombination-cascade matrices for all fine structure levels n(SLJ) up to n <=
10: 98 bound fine structure levels of Ni XXVI with 0 <= l <= 9, 0 <= L <= 11,
1/2 <= J <= 17/2, and 198 levels of Ni XXVII with 0 <= l <= 9, 0 <= L <= 14, 0
<= J <= 10. Total alpha_{RC} for Ni XXVI and Ni XXVII are compared with the
existing values with very good agreement. Total recombination rate coefficients
for the hydrogen-like recombined ion, Ni XXVIII, are also presented. The
calculations are carried out in relativistic Breit-Pauli R-matrix (BPRM)
approximation with inclusion of radiation damping of resonances. With
consideration of all details of the processes, the results, which include the
level specific sigma_{PI} and alpha_{R} calculated for the first time, should
be the most accurate for these ions.Comment: 27 pages, 10 figures, to appear in Astrophys. J. Supplement
Photoionization cross sections of O II, O III, O IV, and O V: benchmarking R-matrix theory and experiments
For crucial tests between theory and experiment, ab initio close coupling
calculations are carried out for photoionization of O II, O III, O IV, O V. The
relativistic fine structure and resonance effects are studied using the
R-matrix and its relativistic variant the Breit Pauli R-matrix (BPRM)
approximation. Detailed comparison is made with high resolution experimental
measurements carried out in three different set-ups: Advanced Light Source at
Berkeley, and synchrotron radiation experiments at University of Aarhus and
University of Paris-Sud. The comparisons illustrate physical effects in
photoionization such as (i) fine structure, (ii) resolution, and (iii)
metastable components. Photoionization cross sections sigma{PI} of the ground
and a few low lying excited states of these ions obtained in the experimental
spectrum include combined features of these states. Theoretically calculated
resonances need to be resolved with extremely fine energy mesh for precise
comparison. In addition, prominent resonant features are observed in the
measured spectra from transitions allowed with relativistic fine structure, but
not in LS coupling. The sigma_{PI} are obtained for ground and metastable (i)
2s^22p^3(^4S^o, ^2D^o, ^2P^o) states of O II, (ii) 2s^22p^2(^3P,^1D,^1S) and
2s2p^3(^5S^o) states of O III, (iii) 2s^22p(^2P^o_J) and 2s2p^2(^4P_J) levels
of O IV, and (iv) 2s^2(^1S) and 2s2p(^3P^o,^1P^o) states of O V. It is found
that resonances in ground and metastable cross sections can be a diagnostic of
experimental beam composition, with potential ap plications to astrophysical
and laboratory plasma environments.Comment: 27 pages, 7 figs., submitted to Phys. Rev. A., text with high
resolution figures at http://www.astronomy.ohio-state.edu/~pradhan/Oions.p
K-shell dielectronic resonances in photoabsorption: differential oscillator strengths for Li-like C IV, O VI, and Fe XXIV
Recently X-ray photoabsorption in KLL resonances of O VI was predicted
[Pradhan, Astrophys.J. Lett. 545, L165 (2000)], and detected by the Chandra
X-ray Observatory [Lee et al, Astrophys. J. {\it Lett.}, submitted].
The required resonance oscillator strengths f_r, are evaluated in terms of
the differential oscillator strength df/de that relates bound and continuum
absorption. We present the f_r values from radiatively damped and undamped
photoionization cross sections for Li-like C,O, and Fe calculated using
relativistic close coupling Breit-Pauli R-matrix method. The KLL resonances of
interest here are: 1s2p (^3P^o) 2s [^4P^o_{1/2,3/2}, ^2P^o_{1/2,3/2}] and 1s2p
(^1P^o) 2s [^2P^o_{1/2,3/2}]. The KLL photoabsorption resonances in Fe XXIV are
fully resolved up to natural autoionization profiles for the first time. It is
demonstrated that the undamped f_r independently yield the resonance radiative
decay rates, and thereby provide a precise check on the resolution of
photoionization calculations in general. The predicted photoabsorption features
should be detectable by the X-ray space observatories and enable column
densities in highly ionized astrophysical plasmas to be determined from the
calculated f_r. The dielectronic satellites may appear as redward broadening of
resonances lines in emission and absorption.Comment: 9 pages, 2 figurs, Phys. Rev. A, Rapid Communication (submitted
Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements. V. Relativistic calculations for Fe XXIV and Fe XXV for X-ray modeling
Photoionization and recombination cross sections and rate coefficients are
calculated for Li-like Fe XXIV and He-like Fe XXV using the Breit-Pauli
R-matrix (BPRM) method. A complete set of total and level-specific parameters
is obtained to enable X-ray photoionization and spectral modeling. The ab
initio calculations for the unified (e + ion) recombination rate coefficients
include both the non-resonant and the resonant recombination (radiative and
di-electronic recombination, RR and DR, respectively) for (e + Fe XXV) -> Fe
XXIV and (e + Fe XXVI) -> Fe XXV. The level specific rates are computed for all
fine structure levels up to n = 10, enabling accurate computation of
recombination-cascade matrices and effective rates for the X-ray lines. The
total recombination rate coefficients for both Fe XXIV and Fe XXV differ
considerably, by several factors, from the sum of RR and DR rates currently
used to compute ionization fractions in astrophysical models. As the
photoionization/recombination calculations are carried out using an identical
eigenfunction expansion, the cross sections for both processes are
theoretically self-consistent; the overall uncertainty is estimated to be about
10-20%. All data for Fe XXIV and Fe XXV (and also for H-like Fe XXVI, included
for completeness) are available electronically.Comment: 31 pages, 10fug
X-ray absorption via K-alpha resonance complexes in oxygen ions
The K-alpha resonance complexes in oxygen ions O I - O VI are theoretically
computed and resonance oscillator strengths and wavelengths are presented. The
highly resolved photoionization cross sections, with relativistic fine
structure, are computed in the coupled channel approximation using the
Breit-Pauli R-matrix method. A number of strong K-alpha resonances are found to
be appreciable, with resonance oscillator strengths f_r > 0.1. The K-alpha
resonance wavelengths of O I-O VI lie in a relatively narrow wavelength range
22 - 23.5 A, and the X-ray opacity in this region should therefore be
significantly affected by K --> L transitions in oxygen. The results should be
useful in the interpretation of soft X-ray spectra observed from Chandra and
XMM-Newton.Comment: Monthly Notices of Roy. Astro. Soc. (in press), 10 pgs. 1 figur
R-Matrix calculations for opacities: I. Methodology and computations
An extended version of the R-matrix methodology is presented for calculation
of radiative parameters for improved plasma opacities. Contrast and comparisons
with existing methods primarily relying on the Distorted Wave (DW)
approximation are discussed to verify accuracy and resolve outstanding issues,
particularly with reference to the Opacity Project (OP). Among the improvements
incorporated are: (i) large-scale Breit-Pauli R-matrix (BPRM) calculations for
complex atomic systems including fine structure, (ii) convergent close coupling
wave function expansions for the (e+ion) system to compute oscillator strengths
and photoionization cross sections, (iii) open and closed shell iron ions of
interest in astrophysics and experiments, (iv) a treatment for plasma
broadening of autoionizing resonances as function of energy-temperature-density
dependent cross sections, (v) a "top-up" procedure to compare convergence with
R-matrix calculations for highly excited levels, and (vi) spectroscopic
identification of resonances and bound \eion levels. The present R-matrix
monochromatic opacity spectra are fundamentally different from OP and lead to
enhanced Rosseland and Planck mean opacities. An outline of the work reported
in other papers in this series and those in progress is presented. Based on the
present re-examination of the OP work, it is evident that opacities of heavy
elements require revisions in high temperature-density plasma sources.Comment: 16 pages, 2 figure
Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements; 4, Relativistic calculations for C IV and C V for UV and X-ray modeling
The first complete set of unified cross sections and rate coefficients are calculated for photoionization and recombination of He- and Li-like ions using the relativistic Breit-Pauli R-matrix method. We present total, unified (e + ion) recombination rate coefficients for (e + C VI ---> C V) and (e + C V \longrightarrow C IV) including fine structure. Level-specific recombination rate coefficients up to the n = 10 levels are also obtained for the first time; these differ considerably from the approximate rates currently available. Applications to recombination-cascade coefficients in X-ray spectral models of K-alpha emission from the important He-like ions is pointed out. The overall uncertainty in the total recombination rates should not exceed 10-20%. Ionization fractions for Carbon are recomputed in the coronal approximation using the new rates. The present (e + ion) recombination rate coefficients are compared with several sets of available data, including previous LS coupling results, and `experimentally derived' rate coefficients. The role of relativistic fine structure, resolution of resonances, radiation damping, and interference effects is discussed. Two general features of recombination rates are noted: (i) the non-resonant (radiative recombination) peak as E,T ---> 0, and the (ii) the high-T resonant (di-electronic recombination) peak
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