Mg-like Ions Research Articles

MulticonfigurationDirac-Hartree-Fock calculations of the forbidden transitionsbetween 3s2 1S0,3s3p 3P0,1,2,1P1states for Mg-like ions

Weak forbidden transitions are of great interest for plasmadiagnostics,but they are sensitive to many factors in theoretical calculations.Inearlier work on the intercombination transition using themulticonfigurationDirac-Hartree-Fock approach for Al+-S4+ ions, a methodwasdeveloped to eliminate the dependence of the transition rates on theoptimization strategy. In this paper, this method is applied to otherLSforbidden transitions. For the LS allowed transitions the transitionenergynormalization can eliminate such a dependence. It is also found thatthe finestructure splittings corrected by the non-relativistic limit offsetsareindependent of optimization strategies and are in good agreement withobservations. For the high-Z ions, the dependence on strategies islessimportant and the calculations for all the transitions (E1, E2, M1,M2) areextended to Z = 92. A general scaling formula is adopted to reducetheextensive variation of the rates with respect to the nuclear charge.

The Spectroscopy of High Spin States for Highly Charged Mg-like Ions

Transitions between high spin core-excited states of highly charged Mg-like Ions (Ti, Fe and Ni) have been investigated using beam-foil spectroscopy. The identification of the 2p53s3p2 5D4–2p53s3p3d 5F5 transition has been aided by calculations and iso-electronic methods.

CIV3 and relativistic quantum defect orbital (RQDO)oscillator strengths in Mg-like ions of astrophysicalinterest

Astrophysically relevant transitions in the Mg-like ions Si III,S V and Fe XV have been studied. A J-dependent CIV3calculation, more extensive than other, earlier ones, has ledto improved accuracy in the calculated energy levels andoscillator strengths. Independent relativistic quantum defect orbitaland multiconfigurational Dirac-Fockcalculations have also been performed. An analysis of theresults is given.

Excitation energies and transition rates in magnesiumlike ions

Excitation energies, transition probabilities, and lifetimes are calculated for 3 l 13l 2-3l 33l 4 electric dipole transitions in Mg-like ions with nuclear charges Z ranging from 13 to 100. Relativistic many-body perturbation theory ~MBPT!, including the Breit interaction, is used to evaluate retarded E1 matrix elements in length and velocity forms. The calculations start from a 1 s 2 2s 2 2p 6 Dirac-Fock potential. First-order perturbation theory is used to obtain intermediate coupling coefficients, and the second-order MBPT is used to determine the matrix elements. The contributions from negative-energy states are included in the second-order E1 matrix elements to achieve agreement between length-form and velocity-form amplitudes. The resulting transition energies and lifetimes are compared with experimental values and with results from other recent calculations.

Measurement of the B+ and Al+ intercombination and Sc12+ forbidden transition rates at a heavy-ion storage ring

Electric-dipole intercombination and forbidden transitions have been optically observed with ions circulating in a storage ring, and atomic lifetimes determined. For the level in the Be-like ion , the measured lifetime of ms corresponds to a transition probability of . This result ties in with experimental work on the neighbouring ion and with recent calculations. For the corresponding level in the Mg-like ion , a lifetime of s (transition rate ) has been measured, corroborating earlier experiments but not the more recent calculations. The transition probability of the M1/E2 transition between the fine-structure levels of the ground state in F-like has been determined as , in agreement with the results of semi-empirically corrected multi-configuration Dirac-Fock calculations.

Electron-impact excitation of Mg-like ions

In a recent paper (1998 J. Phys. B: At. Mol. Opt. Phys. 31 3713-27), we described an intermediate-coupling frame transformation (ICFT) method based on multi-channel quantum defect theory that allows one to generate accurate level-to-level excitation data from unphysical K-matrices calculated in pure LS-coupling. In this paper, we consider ICFT R-matrix close-coupling calculations of effective collision strengths for the Mg-like ions Si2+, Ar6+, Ti10+ and Fe14+. In order to further demonstrate the level of accuracy of this method, we compare our ICFT effective collision strengths with those determined from a full Breit-Pauli R-matrix calculation for Fe14+. We also consider the use of our radiative rates and effective collision strengths in conjunction with a collisional-radiative program to generate line-emission intensity ratios as a function of density and temperature. The full set of energies, radiative rates, and effective collision strengths for these four ions have been made available on the Internet.

Application of the effective valence shell Hamiltonian method to accurate estimation of oscillator strengths and excitation energies of Mg-like ions

The ab initio effective valence shell Hamiltonian (Hυ) method is used to compute the excitation energies and oscillator strengths for resonance transitions in Mg-like ions, as well as their lowest ionization potentials. The computed excitation energies and oscillator strengths from the Hυ method are in excellent agreement with experiment and with the best values from other high level correlated computations, where available. Several previous discrepancies between theory and experiment are now removed. The present work also investigates the dependence of the calculated Hυ oscillator strengths on the nature and choice of the valence orbitals and provides a comprehensive study of the convergence of Hυ calculations with respect to the enlargement of the valence space.

Electron Density Diagnostics for the Solar Upper Atmosphere from Spectra Obtained by SUMER/SOHO

We evaluate the electron density in various solar regions above the limb observed by the Solar Ultraviolet Measurement of Emitted Radiation (SUMER) instrument on SOHO. We find in general good agreement among line ratios from Be-, B-, N-, and Mg-like ions, giving densities of order 108 cm-3.

Large-scale relativistic configuration-interaction calculation of the3s21S0-3s3p1,3P1transition energies in magnesiumlike ions

We have calculated the ${3\mathrm{s}}^{21}$${\mathrm{S}}_{0}$-${3\mathrm{s}3\mathrm{p}}^{1,3}$${\mathrm{P}}_{1}$ transition energies for neutral magnesium and Mg-like ${\mathrm{Ar}}^{6+}$, ${\mathrm{Cu}}^{17+}$, ${\mathrm{Kr}}^{24+}$, and ${\mathrm{Mo}}^{30+}$ 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.

Intercombination lines in highly charged ions

Experimental decay rates for the spin-forbidden ns2npk−1-nsnpk transitions in highly ionized Be-like, B-like, Mg-like and Al-like ions have been obtained by the beam-foil technique. Results are reported for Be-like and B-like Fe22+, Al-like Kr23+, Y26+ and Rh32+ and Mg-like Ni16+, Kr24+, Y27+ Zr28+, Nb29+ and Rh33+. For all ions in the Mg- and Al-like sequences the experimental oscillator strengths are larger than the theoretical predictions and, furthermore, the discrepancy tends to increase with increasing charge state. For both the Be- and B-like ions good agreement is found between theory and experiment. The experimental data presented here was obtained using the RILAC accelerator at RIKEN.

THEORETICAL STUDY OF SPECTRA OF 3-2 TRANSITION AND ITS SATELLITE LINES FOR Ne-LIKE ION OF La

Based on the calculated wavelengths and transition probabilities of n-3 to n-2 transitions for Ne-like, Na-like, Mg-like and Al-like ions of La, in the case of high temperature condition and local thermodynamic equilibrium approximation, we simulated the spectral structure of all the charged states by considering the broadening of individual line and overlapping of a large number of lines in plasma, presented the mean wavelengths and half width of the satellite peaks, interpreted the previous experimental spectral structures, and predicted some new satellite peaks in a larger wavelength region.

Beam-foil studies of highly ionized Nb

We have used the technique of beam-foil spectroscopy to investigate transition probabilities for Na-like Nb 30+ and Mg-like Nb 29+. In the former case the lifetimes of the 3 p 2P 1 2 , 3 2 levels were determined using the ANDC method of cascade correction. For the Mg-like ion we have studied the decay of the 3s3p 3P 1 level which gives rise to a spin-forbidden transition to the 3s 2 1S 0 ground term.

Transitions from Na-like and Mg-like autoionizing levels of multicharged molybdenum ions in an X-pinch plasma

A superdense X-pinch plasma was used to investigate the emission spectra of multicharged molybdenum ions in the spectral region 4.4 – 5.0 Å. A comparison of the experimental data obtained with results of theoretical calculations allowed the identification of a number of spectral lines caused by radiative decay of autoionizing levels of Na- and Mg-like ions. The wavelengths of the identified spectral lines were measured with a relative accuracy of about ±0.001 Å. These measurements provide important tests of radiative emission models of dense plasmas.

A laser-induced fluorescence diagnostic for divertors

It is proposed that the technique of laser-induced fluorescence (LIF) be used to measure local ion velocity distributions in tokamak divertors. Recent theoretical work has indicated the existence of highly populated excited states of many partially stripped ions. These states are often connected to nearby levels by means of visible radiation, allowing useful LIF schemes to be devised. Passive spectroscopic measurements on DIII–D will be presented which verify some of the theoretical predictions. We considered ten Be-like and Mg-like ions and were able to identify over 150 LIF schemes. A possible laser system and experimental layout for the DIII–D tokamak will be presented.

Interacting resonance effects along an isoelectronic sequence: Electron-impact excitation of Mg-like ions.

We have carried out a series of isolated-resonance and R-matrix close-coupling calculations for the electron-impact excitation of the 3${\mathit{s}}^{2}$ $^{1}$S\ensuremath{\rightarrow}3s3p $^{3}$P transition in the Mg-like ions ${\mathrm{Ar}}^{6+}$, ${\mathrm{Ti}}^{10+}$, ${\mathrm{Fe}}^{14+}$, and ${\mathrm{Se}}^{22+}$. We obtain good agreement between the results of the two theoretical methods for three-state calculations, i.e., for the description of just a single Rydberg series of resonances. However, when two or more Rydberg series are present we observe large differences, due to interacting resonance effects, over a wide range of energies for all of the ions examined. For the case of ${\mathrm{Fe}}^{14+}$, we have continued the perturbative studies of interacting resonances of Griffin et al. [Phys. Rev. Lett. 72, 3491 (1994)] and also find the predominant effect to be due to direct configuration interaction between the resonant states rather than being due to their coupling through a common continuum. Our inclusion of these interacting resonance effects removes the major discrepancies between the perturbative and nonperturbative results for ${\mathrm{Fe}}^{14+}$ reported previously [Badnell et al., Phys. Rev. A 48, R2519 (1993)]. This is of importance for the study of indirect processes which are strongly radiation damped, for which the perturbative approach is widely used and the standard formulation of the close-coupling approximation is not applicable.

Atomic data for opacity calculations. XIX. The magnesium isoelectronic sequence

As a contribution to the International Opacity Project, large numbers of multiplet term energies, f-values and photoionization cross sections have been calculated for those Mg-like ions of astrophysical interest: Mg I, Al II, Si III, S V, Ar VII, Ca IX and Fe XV. The present atomic data have been obtained in the close-coupling approximation with a modified version of the RMATRX computer program and asymptotic codes. We briefly discuss the new results, and representative comparisons with previous experimental and theoretical work are carried out. Emphasis is given to the large number of photoionization cross sections reported for the first time. We conclude that the statistical accuracy of the Opacity Project datasets for the Mg isoelectronic sequence is at least as high as that of any other set of data produced with the most elaborate methods available to date.

Relativistic excitation energies and oscillator strengths including core-polarization effects for the intercombination and resonance transitions in Mg-like ions

Excitation energies and oscillator strengths from the 1S0 ground state to the first 3P10 and 1P10 excited states of Mg-like ions are calculated using the multiconfiguration relativistic random-phase approximation including excitation channels from core electrons. The discrepancies between the relativistic random-phase approximation calculation and experimental results for the intercombination excitation energies and oscillator strengths are much reduced. The excitation energies and oscillator strengths for the resonance transition from the multiconfiguration relativistic random-phase approximation are in excellent agreement with experiment. A substantial part of the discrepancy in the 3P10-1P10 separations is resolved when core excitation channels are taken into account.

Breakdown of the isolated-resonance approximation for the electron-impact excitation of positive ions.

We have carried out a series of calculations, the results of which demonstrate the breakdown of the isolated-resonance approximation for the electron-impact excitation of positive ions. The specific results presented here, namely for the 3${\mathit{s}}^{2}$ $^{1}$S\ensuremath{\rightarrow}3s3p $^{3}$P transition in ${\mathrm{Fe}}^{14+}$, are not unique. All of the Mg-like ions that we have investigated so far exhibit the same breakdown. There is no reason to believe either that the Mg-like sequence is unique. This has important implications for the description of resonant processes that are strongly radiation damped, for which the isolated-resonance approximation is widely used and the standard formulation of the close-coupling approximation is not applicable.

Experimental transition probabilities of intercombination transitions in Mg-like and Al-like ions of bromine.

Time-resolved extreme ultraviolet spectra of foil-excited 120-MeV Br ions have been recorded, using a spectrometer equipped with a position-sensitive detector. The spectra near \ensuremath{\lambda}=20--30 nm were measured at times after excitation of up to 6 ns. Prominent in the delayed spectra were the long-lived decays of the intercombination transitions in the Mg-like ${\mathrm{Br}}^{23+}$ and Al-like ${\mathrm{Br}}^{22+}$ ions. The present paper reports on lifetime measurements for intercombination transitions in highly charged bromine ions using such a position-sensitive detector.

A burst model for line emission in the solar atmosphere. II - Coronal extreme ultraviolet lines

view Abstract Citations (34) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS A Burst Model for Line Emission in the Solar Atmosphere. II. Coronal Extreme Ultraviolet Lines Feldman, U. ; Laming, J. M. ; Mandelbaum, P. ; Goldstein, W. H. ; Osterheld, A. Abstract We review the spectroscopic evidence for explosive events - bursts - occurring in solar coronal plasmas. Problems in the interpretation of extreme ultraviolet spectra from Mg-like ions, particularly Fe XV in solar flares can be obviated to certain extent if these ions are produced in an ionizing plasma, with the 3s3p 3P levels being excited directly by inner shell ionization of the Al-like ion. We believe that a similar mechanism should apply to Be-like ions. The excitation following inner shell ionization can become important compared to electron impact excitation at a temperature corresponding to approximately twice the ionization potential of the ion concerned, but to be properly effective, it requires some deviation of the electron distribution function from a Maxwellian. The model we present produces a coronal heating rate from these bursts that is somewhat larger than current estimates. Publication: The Astrophysical Journal Pub Date: October 1992 DOI: 10.1086/171893 Bibcode: 1992ApJ...398..692F Keywords: Emission Spectra; Solar Atmosphere; Solar Corona; Solar Radio Bursts; Ultraviolet Spectra; Line Spectra; Plasma Heating; Solar Flares; Solar Physics; ATOMIC DATA; ATOMIC PROCESSES; PLASMAS; SUN: CORONA full text sources ADS | Related Materials (2) Part 1: 1992ApJ...386..364L Part 3: 1993ApJ...404..799F