Guo Xin Chen

X-ray absorption via Kα resonance complexes in oxygen ions

The Kα 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 Kα resonances are found to be appreciable, with resonance oscillator strengths f r > 0.1. The Kα resonance wavelengths of O I‐O VI lie in the 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

Relativistic Fine Structure and Resonance Effects in Electron-Ion Recombination and Excitation of (e + C IV)

Relativistic close coupling calculations are reported for unified electronic recombination of (e + C IV) including non-resonant and resonant recombination processes, radiative and dielectronic recombination (RR and DR). Detailed comparison of the theoretical unified results with two recent experiments on ion storage rings (Mannervik etal. [1] and Schippers etal. [2]) shows very good agreement in the entire measured energy region 2s -- 2p with 2pnl resonances. The results benchmark theory and experiments to uncertainties of ~15%, and show that the resonant and the background cross sections are not an incoherent sum of separate RR and DR contributions. The limiting values of the DR cross sections, as n ---> infinity, are shown to correspond to those due to electron impact excitation (EIE) at the 2Po_(1/2,3/2) fine structure thresholds, delineated for the first time. The near-threshold 2s 2S_(1/2) - 2p 2Po_(1/2,3/2) EIE cross sections are also compared with recent experimental measurements. The demonstrated threshold fine structure and resonance effects should be of general importance in excitation and recombination of positive ions.

Influence of resonances on spectral formation of x-ray lines in Fe XVII.

New theoretical results from large-scale relativistic close coupling calculations reveal the precise effect of resonances in collisional excitation of x-ray lines of Ne-like Fe XVII. By employing the Breit-Pauli R-matrix method and an 89-level eigenfunction expansion, including up to n = 4 levels, significant resonance enhancement of the collision strengths of forbidden and intercombination transitions is shown. The present results should help resolve long-standing discrepancies; in particular, the present line ratios of three benchmark diagnostic lines 3C, 3D, and 3E at 15.014, 15.265, and 15.456 A, respectively, are in excellent agreement with two independent measurements on electron-beam ion traps. Strong energy dependence in cross sections due to resonances is demonstrated.

Atomic data from the Iron Project - XLIV. Transition probabilities and line ratios for Fe VI with fluorescent excitation in planetary nebulae

Relativistic atomic structure calculations for electric dipole (E1), electric quadrupole (E2) and mag- netic dipole (M1) transition probabilities among the rst 80 ne-structure levels of Fe VI, dominated by congu- rations 3d 3 ; 3d 2 4s, and 3d 2 4p, are carried out using the Breit-Pauli version of the code SUPERSTRUCTURE. Experimental energies are used to improve the accuracy of these transition probabilities. Employing the 80-level collision-radiative (CR) model with these dipole and for- bidden transition probabilities, and Iron ProjectR-matrix collisional data, we present a number of (Fe VI) line ratios applicable to spectral diagnostics of photoionized H II re- gions. It is shown that continuum fluorescent excitation needs to be considered in CR models in order to interpret the observed line ratios of optical (Fe VI) lines in plan- etary nebulae NGC 6741, IC 351, and NGC 7662. The analysis leads to parametrization of line ratios as function of, and as constraints on, the electron density and tem- perature, as well as the eective radiation temperature of the central source and a geometrical dilution factor. The spectral diagnostics may also help ascertain observational uncertainties. The method may be generally applicable to other objects with intensive background radiation elds, such as novae and active galactic nuclei. The extensive new Iron Project radiative and collisional calculations enable a consistent analysis of many line ratios for the complex iron ions 1 .

Relativistic and resonance effects in electron impact excitation of Fe5

Relativistic and resonance effects in electron impact excitation of Fe5+ are investigated using four sets of close-coupling calculations with the R-matrix method. The different eigenfunction expansions for the N-electron target ion consist of eight LS terms, 19 fine structure levels, dominated by the ground configuration 3d3, and 34 LS terms (80 fine structure levels) dominated by additional configurations 3d24s and 3d24p. Relativistic effects are considered in two ways: a full intermediate coupling 19-level Breit-Pauli (BP) approximation, and using the term-coupling coefficients for the target levels. Unlike the lower ionization stages of Fe, it is necessary to consider explicitly the fine structure in the target in order to delineate precisely the positions of autoionizing resonances and detailed resonance structures. Otherwise, the relativistic effects are small, as indicated by the Maxwellian averaged collision strengths from the different approximations. It is found that the presence of particular bound-channel resonances, corresponding to (N + 1)-electron correlation functions in the close-coupling expansion for the (e- + ion) system, has a marked effect on the near-threshold cross sections. General implications of this work are discussed. As these resonances represent real physical features they should not be treated as pseudo-resonances. It is shown that many resonant features are well reproduced by the chosen (N + 1)-electron basis set of correlation functions in the smaller eight-term LS and 19-level BP calculations, as verified by comparison with the 34-term calculation. The present collision strengths, with large resonance structures, differ considerably from earlier calculations without resonances.

Fluorescent Excitation of Spectral Lines in Planetary Nebulae

Fluorescent excitation of spectral lines is demonstrated as a function of temperature-luminosity and the distance of the emitting region from the central stars of planetary nebulae (PNs). The electron densities and temperatures are determined, and the method is exemplified through a detailed analysis of spectral observations of a high-excitation PN, NGC 6741. Fluorescence should also be important in the determination of element abundances. It is suggested that the method could be generally applied to determine or constrain the luminosity and the region of spectral emission in other intensively radiative sources such as novae, supernovae, and active galactic nuclei.