Electron-Ion Dissociative Recombination

The electron-ion dissociative recombination N2H + e is an important loss process for NsH in laboratory plasmas and in interstellar gas clouds [1, 2]. 

The rate constants for the dissociative recombination process are summarized in Table 5. The rate constants, appropriate to plasmas in which the temperatures of the component species (electron, ion, and gas) are equal, are designated and those appropriate to conditions for which the temperature of the electrons (TJ is greater than that of ions and 

N2H decay monitored spectroscopically on several vibration-rotation levels. — Average of the values reported. - Extrapolated from the data in [8]. - For an extrapolated cross section, see text. 

SCF values for the sum of the contributions of the electric moments and the sum of the contributions of the polarizabilities to the interaction energy of the system e -N2H were calculated for different specified positions of the electron [10]. 

When an electron neutralizes a positive ion, the energy released can be dissipated either in photon emission (radiative recombination), or by a third body encounter with the transient excited atom or molecule (three-body recombination) or by the fragmentation of the transient excited molecule (dissociative recombination). Radiative recombination only occurs with a very small probability and three-body recombination only occurs at high pressures or high charge densities, neither of these being appropriate to the atmospheric plasma. It is the dissociative process, exemplified by reactions (5a) and (5b), which is dominant in the ionosphere. In fact, reactions (5a) and (5b) are almost entirely responsible for the loss of ionization in the ionosphere above 85 km altitude (with N2 recombination contributing somewhat) as is readily shown by simple calculations based on laboratory determinations of dissociative recombination coefficients, are, for the dominant molecular ions 02 and NO+. 

Several laboratory groups have contributed to the determination of dissociative recombination coefficients during the last decade or so, but the greatest contribu- 

For are (NO+), however, the situation is not so clear. The 300 K value is well established (are (NO+) = 4.3 x 10-7 cm3 s-1)192, 201 but serious disagreement exists between the Te dependence of are (NO+) obtained from the SA experiment of Huang et al.1951 and the ion trap experiment of Walls and Dunn1991 which give Ore (N0+) T 0,37 and T 0-83 respectively. The recently reported Te dependence by Torr et al.2021 deduced from the night-time AE satellite observations strongly favours that obtained using the ion trap experiment. In a very recent review paper, 

Determination of the dissociative recombination coefficients of cluster ions is very challenging experimentally since it is almost impossible to establish one cluster ion species as the only positive ion in a plasma and so the raw data has to be decon-voluted to obtain are for each of the individual ion species which are present. The only significant amount of data obtained to date has again been obtained using the 

SA-microwave technique104,194, 196) and relates to the H30+ (H20)n and NH4 (NH3)m cluster ion series, although data relating to the reaction  

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Different gases such as CH4, NH3 and H2 have be used in collision cells to reduce isobarie or polyatomic molecular interferences (Niemela et al. 2003). H2 or He is typically used with the ICP-Q-MS employed in the present study (i.e. a Varian 820 ICP-MS). In this instrument, the flow of gas is introduced into the plasma with a double-walled skimmer cone. While the eoUision/reaetion takes place in the plasma, high amounts of electrons are available for electron-ion dissociative recombination to attenuate molecular interferences (Abdelnour and Murphy 2004). All samples were measured both in standard mode (without He) and with the aid of He as a collision gas in conjunction with the ID-ICP-Q-MS for comparative purposes. 

Merged-beam measurements 23-26 have consistently shown that the measured recombination cross section depends on conditions in the ion source. The authors have ascribed the effect to differing vibrational distributions. In one of the later measurements,16 the Hj vibrational state was inferred from the threshold energy for electron-ion dissociative excitation,... 

Figure 4 The ion chemistry of the upper terrestrial atmosphere. Only biomolecular ion-neutral reactions occur, and ion-electron reactions (dissociative recombination) maintain the ionization equilibrium.

CHXNH. Many of the reactions involve radiative association. Dissociative electron-ion recombination then yields neutrals such as CH (methane), C2H OH (ethanol) and CH CN (acetonitrile) [158]. It is often joked... 

The above examples should suffice to show how ion-molecule, dissociative recombination, and neutral-neutral reactions combine to form a variety of small species. Once neutral species are produced, they are destroyed by ion-molecule and neutral-neutral reactions. Stable species such as water and ammonia are depleted only via ion-molecule reactions. The dominant reactive ions in model calculations are the species HCO+, H3, H30+, He+, C+, and H+ many of then-reactions have been studied in the laboratory.41 Radicals such as OH can also be depleted via neutral-neutral reactions with atoms (see reactions 13, 15, 16) and, according to recent measurements, by selected reactions with stable species as well.18 Another loss mechanism in interstellar clouds is adsorption onto dust particles. Still another is photodestruction caused by ultraviolet photons produced when secondary electrons from cosmic ray-induced ionization excite H2, which subsequently fluoresces.42... 

Figure 1 shows three configurations for colliding-beam experiments that have been used for studies of dissociative recombination. These are the inclined-beam apparatus at the University of Newcastle-upon-Tyne,30 the merged-electron—ion-beam experiment (MEIBE) at the University of Western Ontario,31 and an electron-cooler apparatus at the Manne Siegbahn laboratory in Stockholm.32 In the inclined-beam method (Figure la), the ion beam is accelerated to an energy of 30... 

The process of radiative association has recently been reviewed by McMahon16 and by Dunbar.17 The observed neutral molecules are then produced by the dissociative recombination of these product ions with electrons, e.g. ... 

This isomeric form is of interest from an interstellar point of view since the isomer, CH3OH2, is a possible route, via dissociative electron-ion recombination, to the observed methanol.14 A proposed reaction68 leading to this isomer is the radiative association,... 

There has been a consistent motivation for the work presented in this chapter the application to molecular synthesis in interstellar gas clouds (see, for example, Herbst,22 this volume). The species in these regions are detected spectroscopically and are thus automatically isomerically identified. The routes to the observed neutral species consistently involve ion-molecule reactions followed by dissociative electron-ion recombination.18 The first step in this process is to determine whether an isomeric ion can be formed which is likely to recombine to an observed neutral species. The foregoing discussion has shown that whether this occurs depends on the detailed nature of the potential surface. Certainly, this only occurs in some of the cases studied. Much more understanding will be required before the needs of this application are fulfilled. 

Excited states of hydrocarbon molecules often undergo nondissociative transformation, although dissociative transformation is not unknown. In the liquid phase, these excited states are either formed directly or, more often, indirectly by electron-ion or ion-ion recombination. In the latter case, the ultimate fate (e.g., light emission) will be delayed, which offers an experimental window for discrimination. A similar situation exists in liquid argon (and probably other liquefied rare gases), where it has been estimated that -20% of the excitons obtained under high-energy irradiation are formed directly and the rest by recombination (Kubota et al., 1976). 

Binary ion-molecule reactions are indicated by thin arrows (c.t. indicates charge transfer), the radiative association reaction of C+ with H2 is indicated by the thick arrow and the dissociative recombination reactions are indicated by dashed arrows leading to the neutral molecules inside the compound brackets (e indicates free electrons). The molecules indicated in bold are known (observed) interstellar molecules. 

Lastly, we mention one more excitation mechanism that has been observed in molecules. It is well-established that following strong field ionization in atoms and molecules, under certain conditions, the ionized electron can be driven back to the ion core where it can recombine to produce high-harmonic radiation, induce further ionization, or experience inelastic scattering. However, there is also the possibility of collisional excitation. Such excitation was observed in [43] in N2 and O2. In both molecules, one electron is tunnel ionized by the strong laser field. When the electron rescatters with the ion core, it can collisionally ionize and excite the molecular ion, creating either N + or Ol+ in an excited state. When the double ion dissociates, its initial state can... 

In an unsensitized grain, shallow trap states provided by crystal imperfections are important in the trapping of both electrons and holes. Hamilton assumes that the fraction of holes trapped is approximately 1, that is, the concentration of mobile holes is near 0. Nucleation to form silver is inefficient, and a high level of free-electron/trapped-hole recombination occurs. There is a certain probability, however, that a trapped electron will unite with a silver ion to form an atom which may either dissociate back into electron and silver ion or trap another electron and, with a second Ag, form a silver atom pair. This pair is relatively stable and can grow by... 

Among the processes leading to annihilation of free electrons, the most efficient is the dissociative recombination of an electron with a molecular ion. At small electron energies the cross section of such processes exceeds 10 13 cm2. 240 The cross sections for other types of recombination are much smaller. The cross section of dissociative attachment of an electron to a neutral molecule can vary within broad limits from 10 23 to 10 14 cm2, 223 243-244 and is the largest for halogen-containing molecules. 

The ejection of atoms or molecules from the surface of solid in response to primary electronic excitation is referred to as electronically stimulated desorption (ESD) or desorption induced by electronic transitions (DIET). Localization of electronic excitations at the surface of RGS induces DIET of atoms both in excited and in ground states, excimers and ions. Most authors (see e.g. Refs. [8,11,23,30] and references therein) discuss their results on DIET from RGS in terms of three different desorption mechanisms namely (i) M-STE-induced desorption of ground-state atoms (ii) "cavity-ejection" (CE) mechanism of desorption of excited atoms and excimers induced by exciton self-trapping at surface and (iii) "dissociative recombination" (DR) mechanism of desorption of excimers induced by dissociative recombination of trapped holes with electrons. 

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