ELECTRON-ION RECOMBINATION PROCESSES - AN OVERVIEW

Extensive theoretical and experimental studies have been carried out f or the past 20 years on electron-ion recombination processes, as they are applied to the analysis of astrophysical and laboratory plasmas. W e review the basic understanding gained through these efforts, with em phasis on some of the more recent progress made in recombination theor y as the recombining system is affected by time-dependent electric fie lds and plasma particles at low temperature. Together with collisional ionization and excitation processes, recombination is important in de termining ionization balance and excited-state population in non-equil ibrium plasmas. The radiation emitted by plasmas is usually the princi pal medium with which to study the plasma condition, as it is produced mainly during the recombination and decay of excited states of ions i nside the plasma. This is especially true when the plasma under study is not readily accessible by direct probes, as in astrophysical plasma s. Moreover, external probes may sometimes cause undesirable disturban ces of the plasma. Electron-ion recombination proceeds in several diff erent modes. The direct modes include three-body recombination (TBR) a nd one-step radiative recombination (RR), all to the ground- and singl y-excited states of the target ions. By contrast, the indirect resonan t mode is a two-step dielectronic recombination (DR), which proceeds f irst with the formation of doubly-excited states by radiationless exci tation/capture. The resonant states thus formed may relax by autoioniz ation and/or radiative cascades. For more exotic modes of recombinatio n, we consider off-shell dielectronic recombination (radiative DR = RD R), in which an electron capture is accompanied by simultaneous radiat ive emission and excitation of the target ion. Some discussion on atta chment of electrons to neutral atoms, resulting in the formation of ne gative ions, is also given. When resonance states involve one or more electrons in high Rydberg states, presence of an external or intrinsic electric field in the vicinity of the target ions can seriously affec t the ionic states involved and the resulting reaction rates. Such per turbative fields can be intrinsic, as in the case of the plasma ion fi eld, or externally imposed. A proper theoretical treatment of this dif ficult problem is crucial in understanding the recombination process w hich takes place in a field contaminated environment. The simple off-s hell dressing procedure of high Rydberg states by a time-dependent fie ld is reviewed, and the possibility of an anomalously large enhancemen t in the rates, due to the momentum coherence effect (MCE), is discuss ed. The presently available data on recombination rates are summarized , and several important deficiencies and future directions for further research are pointed out. Based on the detailed calculations for a nu mber of cases, several empirical rate formulae for RR and DR processes have been generated to summarize the data for ready applications. As the collection of atoms is cooled to very low temperatures, T < 1 mK s imilar or equal to 10(-8) Ryd, and the bound electrons are ionized by laser irradiation to states of very precisely controlled energies, the prospect for accurate experimental measurements of very-low-energy re combination rates is considered, where the electron temperature can be very low. Therefore, it is of interest to reconsider theoretically so me new phenomena which may occur at such cold environments, in which t he electron de Broglie wavelength can be very large, and both the dens ity and coherent effects, as well as possible field effects, must be p roperly taken into account. Finally, a broader understanding of the va rious recombination processes may be achieved by studying their relati onships to other reactions initiated by electron, ion and photon impac t.