Dielectronic recombination

Recently, it has been found, that some of the doubly excited levels, are influenced by radiative cascades within the Li-like system. This is especially important for levels with small auto-ionization rates Aa and hence low population due to dielectronic recombination. The temperature dependence is taken into account by corrections to (8.2) [18]. 

---

D.R. DeWitt, E. lindroth, R. Schuch, H. Gao, T. Quinteros, W. Zong, Spectroscopy of highly doubly excited states of helium through dielectronic recombination, J. Phys. B 28... 

W. Zong, R. Schuch, E. Lindroth, H. Gao, D.R. DeWitt, S. Asp, et al., Accurate determination of dielectronic recombination resonances with lithium-like argon, Phys. Rev. A 56 (1997) 386. 

S. Mannervik, D. DeWitt, L. Engstrom, J. Lidberg, E. Lindroth, R. Schuch, et al., Strong relativistic effects and natural line widths observed in dielectronic recombination of lithium-like carbon, Phys. Rev. Lett 81 (1998) 313. 

M. Fogle, N. Eklow, E. Lindroth, T. Mohamed, R. Schuch, M. Tokman, Spectroscopic study of Mg-like Ni by dielectronic recombination of stored ions, J. Phys. B 36 (2003) 2563. 

A. Burgess, A general formula for the estimation of dielectronic recombination coefficients in low-density plasmas, Astrophys. J. 141 (1965) 1588. 

G. Kilgus, D. Habs, D. Schwalm, A. Wolf, R. Schuch, N.R. Badnell, Dielectronic recombination from the ground state of heliumlike carbon ions, Phys. Rev. 47 (1993) 4859. 

Z. Altun, A. Yumak, N.R. Badnell, S.D. Loch, M.S. Pindzola, Dielectronic recombination data for dynamic finite-density plasmas, A A 447 (3) (2006) 1165. 

C. Brandau, C. Kozhuharov, Z. Harman, A. Muller, S. Schippers, Y.S. Kozhedub, et al., Isotope shift in the dielectronic recombination of three-electron Nd574, Phys. Rev. Lett. 100 (7) (2008) 073201. 

Investigations on the doubly excited states of two electron systems under weakly coupled plasma have been performed by several authors. Such states usually occur as resonance states in electron atom collisions and are usually autoionizing [225]. Many of these states appear in solar flare and corona [226,227] and contribute significantly to the excitation cross-sections required to determine the rate coefficients for transitions between ionic states in a high temperature plasma. These are particularly important for dielectronic recombination processes which occur in low density high temperature plasma, occurring e.g. in solar corona. Coronal equilibrium is usually guided by the balance between the rates of different ionization and... 

A solid state Si(Li) detector is used to maximise the flux of helium-like vanadium transitions, to minimise contamination from undesirable charge states and to monitor dielectronic recombination transitions. A small amount of pure nitrogen gas is leaked into the trap (injection pressure 5x 10 7Torr) to increase the proportion of lower charge states via evaporative cooling. 

A wide variety of plasma diagnostic applications is available from the measurement of the relatively simple X-ray spectra of He-like ions [1] and references therein. The n = 2 and n = 3 X-ray spectra from many mid- and high-Z He-like ions have been studied in tokamak plasmas [2-4] and in solar flares [5,6]. The high n Rydberg series of medium Z helium-like ions have been observed from Z-pinches [7,8], laser-produced plasmas [9], exploding wires [8], the solar corona [10], tokamaks [11-13] and ion traps [14]. Always associated with X-ray emission from these two electron systems are satellite lines from lithium-like ions. Comparison of observed X-ray spectra with calculated transitions can provide tests of atomic kinetics models and structure calculations for helium- and lithium-like ions. From wavelength measurements, a systematic study of the n and Z dependence of atomic potentials may be undertaken. From the satellite line intensities, the dynamics of level population by dielectronic recombination and inner-shell excitation may be addressed. 

Inner-shell excitation of the Li-like ion core is the second mechanism to populate the doubly excited levels. For the three electron system, the cascade effect between doubly excited levels is negligible compared to dielectronic recombination. This is justified by the fact that for highly charged ions the states with higher n, n > 3, have large initial populations, so therefore the contribution due to the cascade is negligible. The emission of the satellite line is then ... 

The ratio between the w-line, which is predominantly excited by electron collisions (8.1), and the k-satellite, which is populated by dielectronic recombination (8.2), depends on the electron temperature only. The ratio between the w line and the intensity of the collisional excited Li-like satellites (8.5), depends on the density ratio between the Li-like and He-like ions, as the collisional excitation rates for the allowed transitions in the He-like system and in the doubly excited Li-like system are similar. 

The authors are glad to thank many scientists, who have contributed to the results. The TEXTOR team, where the measurements were performed and highly reproducible plasmas were provided, especially to Dr. W. Biel, who did the experiments on the transport properties of TEXTOR, Prof. L. Vainshtein, Dr. A. Urnov and F. Goryaev provided us with the results of atomic data calculation. Dr. N. Badnell trained one of us (O. M.) to get the results on dielectronic recombination using atomic codes. Dr. S. Fritzsche put our attention to the cascades within the Li-like ions and Prof. R. Janev discussed the charge exchange recombination processes. Princeton Plasma Physics Laboratory supported the measurements on TEXTOR, both by cooperation with Dr. M. Bitter, and loan of X-ray detectors. 

State selective dielectronic recombination rate coefficients from Li-like ions to Be-like ions (C, O, Ne, Fe ions) and for carbon L-shell ions have been calculated [10-13]. These data are used to develop collisional-radiative models including dielectronic recombination to excited states [14-17]. The population kinetics of L-shell ions and atoms have been developed and their results have been applied to plasma diagnostics. 

A programme is underway to calculate multi-configuration intermediate coupling dielectronic recombination rate coefficients from the (ground plus) metastable levels of an ion to all possible final states, resolved by level, and/or bundling, appropriate for the GCR modeling. It will cover elements applicable to astrophysics and magnetic fusion viz. He, Li, Be, B, C, N, O, F, Ne, Na, Mg, Al, Si, P, S, Cl, Ar, Ca, Ti, Cr, Fe, Ni, Zn, Kr, Mo and Xe. The first... 

---