Recombination processes, ion

In deriving the kinetic equation describing the arrival of various ionic species at the cathode, it is assumed that the primary species N2 + is formed at the central wire at a constant rate, and during its passage in the direction x perpendicular to the axis its concentration is modified by various reactions. In this treatment both ion diffusion and ion-ion or electron-ion recombination processes are neglected because the geometry of the discharge tube and the presence of an electric field would... 

Braun and Scott (1987) used two-photon ionization of benzene and azulene in n-hexane and followed the e-ion recombination process by monitoring the transient absorption of the electron. The results are not very different from those obtained by the IR stimulation technique. A mean thermalization length of 5.0 nm was inferred at 223 K using a two-photon excitation at 266 nm. Hong and Noolandi s theory was used for the analysis. The absorption technique was... 

The electron-ion recombination processes are also strongly dependent on the type of radiations that ionize a particular system. As it was already mentioned, track effects determine the initial spatial distributions of cations and electrons in irradiated systems. 

One of the most important experimental methods of studying the electron-ion recombination processes in irradiated systems are measurements of the external electric field effect on the radiation-induced conductivity. The applied electric field is expected to increase the escape probability of geminate ion pairs and, thus, enhance the number of free ions in the system, which will result in an enhanced conductivity. 

In the limit of an infinitely long mean free path, the bulk electron-ion recombination may be described using the energy diffusion model [43,44]. This model is especially relevant to the electron-ion recombination processes in the gas phase. 

Comparative Aspects of Electron-Ion Recombination Processes in the Gas and Condensed Phases... 

In the gas phase at one to several atmospheric pressures, electron-ion recombination processes have been studied with a pulse-radiolysis method, and observed recombination rate constants kj are expressed by,... 

Absorption due to main intermediates such as polymer cation radicals and excited states, electrons, and alkyl radicals of saturated hydrocarbon polymers had not been observed for a long time by pulse radiolysis [39]. In 1989, absorption due to the main intermediates was observed clearly in pulse radiolysis of saturated hydrocarbon polymer model compounds except for electrons [39,48]. In 1989, the broad absorption bands due to polymer excited states in the visible region and the tail parts of radical cation and electrons were observed in pulse radiolysis of ethylene-propylene copolymers and the decay of the polymer radical cations were clearly observed [49]. Recently, absorption band due to electrons in saturated hydrocarbon polymer model compounds was observed clearly by pulse radiolysis [49] as shown in Fig. 2. In addition, very broad absorption bands in the infrared region were observed clearly in the pulse radiolysis of ethylene-propylene copolymers [50] as shown in Fig. 3. Radiation protection effects [51] and detailed geminate ion recombination processes [52] of model compounds were studied by nano-, pico-, and subpicosecond pulse radiolyses. 

Y. Hahn, Electron-ion recombination processes in plasmas, in R. Janev (Ed.), Atomic and Molecular Processes in Fusion Edge Plasmas, Plenum Publications Corp., New York, 1995, p. 91. 

The microwave-detection method has been developed [141] for the study of ionic species and their reactions in nonpolar liquids on a nanosecond timescale, and relies on the fact that microwaves are attenuated in weakly conducting media. It is very useful, for example, for the study of geminate recombination of radical ions in liquid hydrocarbons. It is also more suitable than either optical or D.C. conductivity methods for the study of homogeneous ion recombination processes where problems in data analysis can arise from underlying absorptions and distortion of the kinetics due to separation of the ions, respectively. 

Paulsen et al. (1972) developed an optical model for vibrational relaxation in reactive systems. Only collinear atom-diatom collisions were considered, i.e. impact parameter dependencies were omitted. The model was applied to vibrational relaxation of electronically excited I2 in inert gases, in which case dissociation of I2 is responsible for flux loss. Olson (1972) used an absorbing-sphere model for calculating integral cross sections of ion-ion recombination processes A++B ->A + B + AE, with A or B atoms or molecules. He employed the Landau-Zener formula to obtain a critical crossing distance Rc, and assumed the opacity to be unity for distances... 

We have reeently made further detailed FA optieal studies of proeess 27 and the following ion-ion recombination processes ... 

Tladiation chemists have been aware for about 15 years that the presence of dilute solutes in liquid hydrocarbons can change the course of radiation chemical reactions by other than the normally expected secondary radical reactions. For example, Manion and Burton (40) in early work on the radiolysis of benzene-cyclohexane solutions, drew attention to the possibility of energy transfer from solvent to solute. Furthermore, it is known that in hydrocarbon solvents certain solutes are capable of capturing electrons, thus interfering with the normal ion-recombination process (14, 20, 65, 72). Though ionic products can be observed readily in hydrocarbon glasses [e.g., (19, 21)] demonstration of effects which can be specifically ascribed to electron capture in the liquid state has been elusive until recently. Reaction of positive ions prior to neutralization can play an important role as demonstrated recently by studies on... 

Electron-Ion and Ion-Ion Recombination Processes, M. R. Flannery Studies of State-Selective Electron Capture in Atomic Hydrogen by Translational Energy Spectroscopy, H. B. Gilbody Relativistic Electronic Structure of Atoms and Molecules, I. P. Grant The Chemistry of Stellar Environments,... 

To ensure that the ion recombination process was the rate limiting step, the experimental conditions were carefully chosen. The hot electrons, e were thermalized by both xenon and SFg, and this process occurred typically within the duration of the electron pulse. The thermal electrons are rapidly captured by SFg, k = 2.27 X 10 cm s [72], to form the molecular anion, SF, which was... 

Figure 2. Ion recombination processes. Schematic of electron trapping in heterogeneous polymer blend near domain interface.

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 cross section of the ion-ion recombination process can then be presented as... 

Ion-ion recombination processes Rate coefficients, third order Rate coefficients, second order, 1 atm... 

The chain of reactions can be accelerated by vibrational excitation of molecules. The typical reaction time is about 0.1 ms and is much faster than ion-ion recombination (1-3 ms), which determines the termination of the chain. As the negative cluster size increases, the probability of reactions with the vibrationally excited molecules decreases because of an effect of vibrational-translational (VT) relaxation on the cluster surface. When the particle size reaches a critical value (about 2 nm at room temperature) the cliain reaction of cluster growth becomes much slower and is finally stopped by the ion-ion recombination process. The typical time for 2 nm particle formation by this mechanism is about 1 ms at room temperature. A critical temperature effect on particle growth is partially due to VT relaxation, which depends exponentially on translational gas temperature according to the Landau-Teller effect (Section 2.6.2). Even a small increase of gas temperature results in a reduction of the vibrational excitation level and decelerates the cluster growth. 

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