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 fr > 0.1. The Kα resonance wavelengths of O i–O vi lie in the relatively narrow wavelength range 22–23.5 Å, 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.
Time‐dependent numerical simulations of the Kα complex of Fe xxv are carried out as a function of temperature–density–radiation field variations in high‐temperature astrophysical and laboratory plasmas. In addition to several well‐known features, the transient and steady‐state spectra reveal the effects due to (a) time‐dependent thermal and non‐thermal radiation fields, (b) photo‐ and collisional excitation and ionization, and (c) high densities, on the ‘quartet’ of principal w, x, y, z lines, and dielectronic satellites. The highly detailed models show precisely how, assuming a temporal–temperature correlation, the X‐ray intensity varies between 6.6 and 6.7 keV and undergoes a ‘spectral inversion’ in the w and z line intensities, characterizing an ionization‐ or a recombination‐dominated plasma. The dielectronic satellite intensities are the most temperature‐dependent features, but insensitive to density variations, and significantly contribute to the Kα complex for T < 6.7 keV leading to asymmetric profiles. The 6.7‐keV Kα complex should be a potential diagnostic of X‐ray flares in active galactic nuclei, afterglows in gamma‐ray bursts, and other non‐equilibrium sources with the high‐resolution measurements possible from the upcoming mission Astro‐E2. It is also shown that high electron densities attenuate the line intensities in simulations relevant to laboratory plasmas, such as in inertial confinement fusion, laser, or magnetic Z‐pinch devices.
Synchrotron X-ray diffraction and neutron scattering studies are performed on As-Se glasses in two states: as-prepared (rejuvenated) and aged for ~23 years. The first sharp diffraction peak (FSDP) obtained from the structure factor data as a function of composition and temperature indicates that the cooperative processes that are responsible for structural relaxation do not affect FSDP. The results are correlated with the composition dependence of the complex heat capacity of the glasses and concentration of different structural fragments in the glass network. The comparison of structural information shows that density fluctuations, which were thought previously to have a significant contribution to FSDP, have much smaller contribution than the cation-cation correlations, presence of ordered structural fragments or cage molecules.
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