Electron - ion recombination processes - an overview

Abstract

Extensive theoretical and experimental studies have been carried out for the past 20 years on electron-ion recombination processes, as they are applied to the analysis of astrophysical and laboratory plasmas. We review the basic understanding gained through these efforts, with emphasis on some of the more recent progress made in recombination theory as the recombining system is affected by time-dependent electric fields and plasma particles at low temperature. Together with collisional ionization and excitation processes, recombination is important in determining ionization balance and excited-state population in non-equilibrium plasmas. The radiation emitted by plasmas is usually the principal medium with which to study the plasma condition, as it is produced mainly during the recombination and decay of excited states of ions inside the plasma. This is especially true when the plasma under study is not readily accessible by direct probes, as in astrophysical plasmas. Moreover, external probes may sometimes cause undesirable disturbances of the plasma.Electron-ion recombination proceeds in several different modes. The direct modes include three-body recombination (TBR) and one-step radiative recombination (RR), all to the ground-and singly-excited states of the target ions. By contrast, the indirect resonant mode is a two-step dielectronic recombination (DR), which proceeds first with the formation of doubly-excited states by radiationless excitation/capture. The resonant states thus formed may relax by autoionization and/or radiative cascades. For more exotic modes of recombination, we consider off-shell dielectronic recombination (radiative DR = RDR), in which an electron capture is accompanied by simultaneous radiative emission and excitation of the target ion. Some discussion on attachment of electrons to neutral atoms, resulting in the formation of negative 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 affect the ionic states involved and the resulting reaction rates. Such perturbative fields can be intrinsic, as in the case of the plasma ion field, or externally imposed. A proper theoretical treatment of this difficult problem is crucial in understanding the recombination process which takes place in a field contaminated environment. The simple off-shell dressing procedure of high Rydberg states by a time-dependent field is reviewed, and the possibility of an anomalously large enhancement in the rates, due to the momentum coherence effect (MCE), is discussed.

The effects of space charge on contrast in images obtained using the environmental scanning electron microscope

“…Conversely, positive ions drift to the sample surface at a much lower velocity and are therefore more likely to form a space charge. Positive ions can deionize by recombining with electrons during contact with the specimen, the stage, and the chamber walls, and with negative ions and free electrons in the gas (Hahn 1997;Marković et al 1993Marković et al , 1994Marković et al , 1996Nasser 1971;von Engel 1965). In water vapor, the negative ion concentration is negligible (von Engel 1965).…”

“…This can be understood in terms of the short interaction period between a highly mobile electron and a low mobility ion. Free electron-ion recombination rates measured and calculated for a wide range of ionic species (Hahn 1997, Nasser 1971 indicate that this deionization mechanism is only appreciable for slow, low energy (0 < ∼0.5eV) electrons. Electron-ion recombination is therefore neglected in the following discussion.…”

Level-resolved quantum statistical theory of electron capture into many-electron compound resonances in highly charged ions

“…Experimentally, much progress has been made due to the use of ion storage rings and electron-beam ion traps (EBITs) [21][22][23]. On the theory side, a number of computational approaches have been used successfully to describe DR for many simpler ions and to produce data for plasma modelling (see [24][25][26][27][28][29][30][31][32][33][34] and references therein). For more complex targets such as Au 25+ , U 28+ , or W 20+ considered in this work, conventional DR calculations underestimate the measured recombination rates, particularly at low incident-electron energy [17,35].…”

Multiphoton radiative recombination of electron assisted by a laser field

“…Therefore the emitted photon spectrum represents a sequence of equidistant lines separated by the laser frequency ω. The recent review by Hahn [8] on the electron recombination mentions only one, very special version of LAR process, namely one-photon LAR when the laser is tuned in resonance with the energy of free-bound electron transition, and only emission of photons with this particular energy is considered. The study of this special case was initiated quite long ago [9][10][11][12][13] and remains active in connection with the processes in the storage rings [14][15][16][17][18][19][20], formation of positronium [9] and antihydrogen [21][22][23] and even with possible cosmological manifestations [24].…”