Fluorescence of iron ions induced by an X-ray laser allows the relative oscillator strength for Fe xvii emission to be determined; it is found to differ by 3.6σ from the best quantum mechanical calculations, suggesting that the poor agreement between prediction and observations of the brightest Fe xvii line is rooted in the quality of the underlying atomic wavefunctions used in the models. The interpretation of some of the spectral data from the Chandra and XMM-Newton orbiting X-ray missions has been complicated by discrepancies between theory and observation involving the emission lines from the highly charged Fe16+ ion, also known as Fe XVII. Specifically, the intensity of the strongest Fe XVII line, one of the brightest X-ray emissions from galaxies and stars, is generally weaker than predicted. Sven Bernitt et al. report the results of laboratory experiments in which a target of iron ions was fluoresced with femtosecond X-ray pulses from a free-electron laser. They find a relative oscillator strength that differs by 3.6σ from the best quantum mechanical calculations, suggesting that the poor agreement is rooted in the calculations of the underlying atomic dynamics and that the current astrophysical models are not at fault. Highly charged iron (Fe16+, here referred to as Fe xvii) produces some of the brightest X-ray emission lines from hot astrophysical objects1, including galaxy clusters and stellar coronae, and it dominates the emission of the Sun at wavelengths near 15 angstroms. The Fe xvii spectrum is, however, poorly fitted by even the best astrophysical models. A particular problem has been that the intensity of the strongest Fe xvii line is generally weaker than predicted2,3. This has affected the interpretation of observations by the Chandra and XMM-Newton orbiting X-ray missions1, fuelling a continuing controversy over whether this discrepancy is caused by incomplete modelling of the plasma environment in these objects or by shortcomings in the treatment of the underlying atomic physics. Here we report the results of an experiment in which a target of iron ions was induced to fluoresce by subjecting it to femtosecond X-ray pulses from a free-electron laser4; our aim was to isolate a key aspect of the quantum mechanical description of the line emission. Surprisingly, we find a relative oscillator strength that is unexpectedly low, differing by 3.6σ from the best quantum mechanical calculations. Our measurements suggest that the poor agreement is rooted in the quality of the underlying atomic wavefunctions rather than in insufficient modelling of collisional processes.

/pdf/an-unexpectedly-low-oscillator-strength-as-the-origin-of-the-k551kallrp.pdf

An unexpectedly low oscillator strength as the origin of the Fe xvii emission problem

Using a combined configuration interaction and many-body perturbation theory approach, we calculate the level energies of 1s22lq and 1s22lq-13l' (1 ≤ q ≤ 8) complexes in iron and nickel ions. Comparison of 1s22lq energies with experimental results shows that the calculation is accurate to within ~0.15 eV for these levels. The accuracy of 1s22lq-13l' energies can be slightly worse, at ~0.5 eV. The theoretical wavelengths of 2→3 transitions of iron ions are compared with the published experimental measurements, and discrepancies are found to be mostly within ±10 mA. Similar accuracy is expected for wavelengths of nickel ions. Such accuracy is a significant improvement over that obtainable with standard configuration interaction methods. A complete table of 2l → 3l' transition wavelengths of all L-shell ions of iron and nickel is presented.

http://iopscience.iop.org/article/10.1086/425915/pdf

Wavelengths of 2l-->3l' Transitions in L-Shell Ions of Iron and Nickel: A Combined Configuration Interaction and Many-Body Perturbation Theory Approach

http://www.jspf.or.jp/JPFRS/PDF/Vol7/jpfrs2006_07-278.pdf

Relativistic many-body calculations of multipole (E1, M1, E2, M2, E3, and M3) transition wavelengths and rates between 3l−14l′ excited and ground states in nickel-like ions

A relativistic many-body method is developed to calculate energy and transition rates for multipole transitions in many-electron ions. This method is based on relativistic many-body perturbation theory (RMBPT), agrees with MCDF calculations in lowest-order, includes all second-order correlation corrections and includes corrections from negative energy states. Reduced matrix elements, oscillator strengths, and transition rates are calculated for electric-multipole (dipole (E1), quadrupole (E2), and octupole (E3)) and magnetic-multipole (dipole (M1), quadrupole (M2), and octupole (M3)) transitions between 3l5l' excited and ground states in Ni-like ions with nuclear charges ranging from Z = 30 to 100. The calculations start from a 1s22s22p63s23p63d10} Dirac-Fock potential. First-order perturbation theory is used to obtain intermediate-coupling coefficients, and second-order RMBPT is used to determine the matrix elements. A detailed discussion of the various contributions to the dipole matrix elements and energy levels is given for nickellike tungsten (Z = 74). The contributions from negative-energy states are included in the second-order E1, M1, E2 M2, E3, and M3 matrix elements. The resulting transition energies and transition rates are compared with experimental values and with results from other recent calculations. These atomic data are important in modeling of M-shell radiation spectra of heavy ions generated in electron beam ion trap experiments and in M-shell diagnostics of plasmas.

https://arxiv.org/pdf/physics/0605009.pdf

Multipole (E1, M1, E2, M2, E3, M3) transition wavelengths and rates between 3l5l' excited and ground states in nickel-like ions

Energies of the ground and a number of even- and odd-parity excited states of multi-valence-electron ions have been computed by a computational method in the framework of relativistic multireference many-body perturbation theory Relativistic multireference perturbation calculations are reported for the ground and over 80 low-lying odd- and even-parity excited states of siliconlike argon (Ar{sup 4+}) and aluminumlike iron (Fe{sup 13+}) to demonstrate the unprecedented accuracy of the method The theory deviates from experiment by less than 02% for all but a few excited levels in siliconlike argon For the more highly ionized aluminumlike iron, the deviations are reduced to within 006% Theoretical magnetic dipole and electric quadrupole transition rates of the lowest-lying {sup 2}P{sub 3/2}{sup o} state of aluminumlike Fe and Mn are evaluated, and lifetimes are compared with a recent ion trap experiment

Relativistic multireference many-body perturbation-theory calculations on the multiple openshell states in siliconlike Ar and aluminumlike Fe ions

Excitation energies for $2l\ensuremath{-}{3l}^{\ensuremath{'}}$ hole-particle states of Ne-like ions are determined to second order in relativistic many-body perturbation theory (MBPT). Reduced matrix elements, line strengths, and transition rates are calculated for electric-dipole $(E1),$ magnetic-quadrupole $(E2),$ magnetic-dipole $(M1),$ and magnetic-quadrupole $(M2)$ transitions in Ne-like ions with nuclear charges ranging from $Z=11$ to 100. The calculations start from a ${1s}^{2}{2s}^{2}{2p}^{6}$ closed-shell Dirac-Fock potential and include second-order Coulomb and Breit-Coulomb interactions. First-order many-body perturbation theory (MBPT) is used to obtain intermediate-coupling coefficients, and second-order MBPT is used to determine the matrix elements. Contributions from negative-energy states are included in the second-order $E1,$ $M1,$ $E2,$ and $M2$ matrix elements. The resulting transition energies are compared with experimental values and with results from other recent calculations. Trends of $E1,$ $E2,$ $M1,$ and $M2$ transition rates as functions of nuclear charge Z are shown graphically for all transitions to the ground state.

/pdf/electric-dipole-electric-quadrupole-magnetic-dipole-and-38wumcyocf.pdf

I. Murakami

Papers

Electric-dipole, electric-quadrupole, magnetic-dipole, and magnetic-quadrupole transitions in the neon isoelectronic sequence