Electron-ion recombination of molecular

COMPLEX FORMATION IN ELECTRON-ION RECOMBINATION OF MOLECULAR IONS... 

COMPLEX FORMATION IN ELECTRON-ION RECOMBINATION OF MOLECULAR IONS R. Johnsen and J. B. A. Mitchell 49... 

NO Synthesis in Non-Thermal Plasma Provided by Positive Ions and Electronically Excited Molecules. Estimate the energy cost of NO molecule formation in air at non-thermal plasma conditions produced by each of the three following processes (1) dissociation of molecular nitrogen through its electronic excitation by direct electron impact (2) electron-ion recombination of positive molecular nitrogen ion and (3) ion-molecular reactions of... 

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. 

A survey is given of the theoretical and experimental studies of electron-ion recombination in condensed matter as classified into geminate and bulk recombination processes. Because the recombination processes are closely related with the magnitudes of the electron drift mobility, which is largely dependent on molecular media of condensed matter, each recombination process is discussed by further classifying it to the recombination in low- and high-mobility media. 

As meteoric material is evaporated, a number of high energy gas-phase collision processes occur. These processes, examples of which are listed in Fig. 1, involve both collisions between neutrals and neutrals and ions. Metal atoms, Me, are abundant in all meteoric bodies, and play an important role for several reasons (i) Metal atoms and ions frequently have low lying excited states with high oscillator strengths and are, therefore, easily identified and traceable (ii) Metal atoms have low ionization potentials and can be ionized fairly efficiently in high velocity neutral collisions (iii) Atomic metal ions have very long lifetimes with respect to ion-molecule reactions and electron-ion recombination compared with molecular ions, which rapidly dissociatively recombine with electrons. 

The metal atoms of the neutral metal layers are subject to charge transfer ionization by the principal molecular ions of the ionospheric E-region, NO+ and 02" . The highly stable atomic metal ions are either transported to higher altitudes, where they can undergo electron-ion recombination, or they can be removed by three-body association reactions with atmospheric molecules at lower altitudes, such as N2 ... 

The fastest electron neutrahzation mechanism in molecular gases, or in the presence of molecular ions, is dissociative electron-ion recombination ... 

The recombination mechanism (2-35) is quite fast and plays the major role in molecular gases. Reaction rate coefficients for most of the diatomic and triatomic ions are on the level of 10 cm /s. For some important molecular ions, the kinetic information can be found in Table 2-2. In a group of similar ions, like molecular ions of noble gases, the recombination rate coefficients increase as the number of internal electrons increases recombination of KrJ and XeJ is about 100 times faster than that of helium. 

The three-body recombination process (2-37) is the most important one in high-density quasi-equilibrium plasmas. Concentrations of molecular ions are very low in this case (because of thermal dissociation) for the fast mechanism of dissociative recombination described earlier, and the three-body reaction dominates. The recombination process starts with the three-body capture of an electron by a positive ion and formation of a highly excited atom with a binding energy of about. This highly excited atom then gradually loses energy in electron impacts. The three-body electron-ion recombination process (2-37) is a reverse one with respect to the stepwise ionization (see Section 2.1.7). For this reason, the rate coefficient of the recombination can be derived from the stepwise ionization rate coefficient kl (2-25) and from the Saha thermodynamic equation for ionization/recombination balance (see Chapter 3) ... 

The radiative electron-ion recombination (2-38) is also a relatively slow one, because it requires a photon emission during a short interval of the electron-ion interaction. This type of recombination can play a major role only in the absence of molecular ions, and the plasma density is quite low when three-body mechanisms are suppressed. Cross sections of radiative recombination are usually about 10 cm. Rate coefficients can be estimated (Zeldovich Raizer, 1966) as a function of electron temperature ... 

Some ion-molecular reactions were already discussed earlier. Thus, the positive ion conversion A+ -I- B -I- M AB+ + M was considered in Section 2.2.2 as a preliminary stage of the dissociative electron-ion recombination. Ion-molecular reactions not only make a contribution in the balance of charged particles but also provide plasma-chemical processes by themselves. Ion-cluster growth in dusty SiH4 plasmas and ion-molecular chain reactions of SO2 oxidation in air during exhaust gas cleaning are good relevant examples, which will be discussed later in the book. 

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