Atomic Physics of He-Like Spectra

The spectra are dominated by triplet and singlet transitions in He-like ions and associated satellite transitions in the Li- and Be-like system. The spectrum of He-like argon and iron ions is shown in Fig. 8.2. 

The k and j satellites are the strongest dielectronic satellites to the He-like lines, the q, r, s and t satellites have strong contributions due to inner-shell excitation from the Li-like ground state. Besides the dominant collisional excitation of He-like ions in the ground state, recombination processes (radiative, dielectronic and charge exchange) of H- and He-like ions, inner-shell excitation of the Li-like ions and, in the case of the z line, also inner-shell ionization process contribute to the intensity of the He-like lines. We will discuss these processes in detail. 

The role of cascades is different for the singlet and the triplet system and most noticeable for the forbidden transition ls2(1S0)-ls2s(3Si) (z line). The upper level of the resonance line w is weakly affected by cascades this suggests to use the w line as the reference line in the interpretation of the spectra. Another reason is the high line intensity with respect to all other transitions. Among the transitions in the n = 2 levels one has to take into account An = 0 dipole transitions, as well as the ls2s(1So) to the ground state for the description of the spectra. The line emission due to excitation is expressed as 

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