Dielectronic satellite spectra of heliumlike iron and nickel from the unified recombination method

It is shown that resonances in the unified recombination cross sections correspond directly to dielectronic satellite spectra. Positions, intensities, and profiles of satellite lines may be obtained accurately using the Breit-Pauli R-matrix method and the unified theory of total electron-ion recombination. The intrinsic dielec- tronic resonance strength corresponding to autoionizing resonances is defined in analogy with measured quan- tities, and with resonance oscillator strengths. Satellite intensities then depend extrinsically on dynamical parameters in the plasma and may be obtained at any temperature in a straightforward manner from the resonance strengths. Profiles of dielectronic satellite lines of He-like Fe XXV and Ni XXVII are obtained in an ab initio manner. Detailed calculations also exhibit interference effects in resonance profiles delineated at high resolution. Although interference among resonances is not important for resonance strengths of the highly charged ions Fe XXV and Ni XXVII it might be significant for lower ionization states of heavy elements in general. The resonance identitification scheme for unified recombination cross sections is described, and applied to obtain all known KLL satellites of Fe XXV and Ni XXVII. Dielectronic satellite strengths are compared with earlier works and significant differences and similarities are discussed. I. INTRODUCTION The dielectronic recombination process manifests itself through the radiative decay of an autoionizing state of the e , ion system. The resonances are formed from doubly ex- cited states following electron impact excitation of a dipole core transition and capture of the incident electron. These resonances appear as "satellite" lines at energies somewhat lower than the energy of the principal dipole core transition 1,2, and are otherwise indistinguishable from ordinary emission lines. X-ray spectra of highly charged ions exhibit dielectronic satellites which, in terms of wavelengths, are at longer wavelengths or "redward" than the principal line of the 1s 2 1 S0 1 s2p 1 P 1 o transition we eschew the phrase "resonance line" common in the literature to avoid obvious confusion. In particular, H-like and He-like ions of high-Z elements have strong satellite lines owing to large radiative decay rates of core transitions which compete effectively with typical autoionization rates A a 10 13 -1 0 14 s 1 . The dielectronic satellites are sensitive plasma diagnostics of electron temperature since they are excited only by elec- trons at precise resonance energies, in contrast to bound- bound core transitions excited by all electrons with energies