Recombination of atoms and radicals

Lifetimes of free atoms and radicals account for the degree of interaction of these particles with an ambient medium and with each other. Due to high reaction capability of active particles in gaseous and, especially, in liquid media, their lifetimes are rather small. In gaseous phase, at small pressures these lifetimes are determined by heterogeneous recombination of these particles on vessel walls and by interaction of these particles with an adsorbed layer. At high gas pressures, the lifetimes are determined by bulk recombination and chemical interaction with ambient molecules. 

According to [ 1 ], in the case of recombination of atoms and radicals governed by the first-order kinetics, the radicals concentration distribution over the height A in a cylindrical vessel can be written as 

For surface recombination governed by the second-order kinetics, this quantity is given by 

As is seen from the above formulas, in the case of heterogeneous recombination of particles governed by the first-order kinetics, absolute values of y determined solely by the distribution of relative concentration of particles along the axis of the cylindrical vessel. For heterogeneous recombination of particles governed by the second-order kinetics, a knowledge of the absolute concentration of these particles for certain h is also required. 

Numerous studies show that, usually, heterogeneous recombination of free atoms and radicals is governed by the first-order kinetics. Thus, to determine y need only to find the dependence v = f(h). To do this, we may use the distribution of relative values v = (,da/di)  

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Chapter 4 deals with several physical and chemical processes featuring various types of active particles to be detected by semiconductor sensors. The most important of them are recombination of atoms and radicals, pyrolysis of simple molecules on hot filaments, photolysis in gaseous phase and in absorbed layer as well as separate stages of several catalytic heterogeneous processes developing on oxides. In this case semiconductor adsorbents play a two-fold role they are acting botii as catalysts and as sensitive elements, i.e. sensors in respect to intermediate active particles appearing on the surface of catalyst in the course of development of catal rtic process. 

The method of catalytic recombination has been developed for the detection of hydrogen atoms and the measurement of their concentrations in various flames such as H2, CO, C2H2. It is based on the fact that the different catalytic activity of various compounds with respect to surface recombination of atoms and radicals is very specific. The mixed oxide Zn0-Cr203 is such a catalytic compound stimulating preferential recombination of hydrogen atoms. When introduced into the flame zone (as a thin film deposited on the surface of a quartz capillary), this catalyst heats up as a result of the recombination H + H -> H2 [238]. 

One of the reasons for the often observed low quantum yields lies first of aU in the low rates of secondary processes, which make possible a competing deactivation of primary active centers either by their consumption outside the reaction (e.g. by recombination of atoms and radicals) or by loss of excitation energy (by emission or in collisions with surrounding molecules). 

Finally, vibrationally and rotationally excited molecules are also formed by recombination of atoms and radicals. 

Fast activationless reactions, such as recombination of atoms and radicals, of course, occur more slowly in liquid than in gas because they are limited by the rate of particle self-diffusion, and diffusion in liquid occurs more slowly than in gas. Therefore, it is of interest to compare slow reactions, which are not limited by diffusion in liquid, to those with rate constants A < 1 o l/(mol s) in the gas phase. As we will see further, the solvation effects and formation of molecular complexes influence strongly on the chemical reaction in liquid. Since solvation is absent ftom the gas phase, for the correct comparison we have to consider reactions in which at least one reactant is a nonpolar particle, for example, hydrocarbon. Reactions of radicals with nonpolar C—H bonds are most suitable for this comparison. The data on such... 

Later experiments by Kokochashvili showed that HCL also has some, albeit weaker, flegmatizing effect in CO combustion. The experiments show that the action of chlorine-containing compounds is due not only to hydrogen bonding, but also to participation of atoms and radicals in combustion recombination processes. Note made in proof (1943— Editor s note). 

Due to this production of free radicals, pyrolysis reactions exhibit pronounced sensid-vity to the geometry of the reactor, and the walls tend to favor the recombination of atoms and intermediate light radicals. 

Reverse processes, i.e. recombination (association) reactions of atoms and radicals with each other or with stable molecules, present a similar dependence of the reaction kinetics on pressure and temperature. The scheme of an recombination (association) reaction also comprises three elementary steps... 

Chapter 1 deals with the kinetics of the dissociation of diatomic molecules and the recombination of atoms, and Chapters 2 and 3 with the reactions of atoms and radicals with molecules, abstraction (metathetical) processes and addition to double and triple bonds. Data for the reactions of metal atoms with a variety of inorganic, organic and metal organic compounds, derived from sodium flame and molecular beam techniques, are discussed in Chapter 4 and rapid substitution at labile metal ions in solution in Chapter 5. The theory of, and the experimental results for, ion-molecule reactions, i.e. chemical processes resulting from binary collisions of positive or negative ions with neutral molecules, are discussed in Chapter 6 and the reactions of solvated electrons in Chapter 7. 

Radiative recombination is a very slow process. It has been deduced from experimental data that the probability P,. of a bromine-atom radiative recombination per collision is of the order of 10" [235]. However, despite this value, radiative recombination is manifested in many low-density systems and sometimes plays the crucial role in chemical transformations occurring in such systems. The continuous luminescence spectra observed for various flames are undoubtedly due mostly to recombination processes, i.e. to radiative stabilization of quasi-molecules formed by collisions of atoms and radicals with each other or with molecules present in the combustion zone (see e.g. [153, 359]). Radiative recombination determines the rate of the formation of molecules in interstellar clouds [102, 514] and the spectra of gas discharge in molecular gases [89, 472]. 

The reactions of free atoms and radicals generated in primary processes as well as in fast ion-molecule reactions and in reactions involving excited particles also have to be relatively fast in order to overtake the neutralization process. Moreover, other atoms and radicals are generated by neutralization. Since the rate constants of atom and radical recombination are by several orders lower than the neutralization constants, a large number of the reactions of radicals with molecules, and atom and radical recombination as such, obviously represent the last processes in the sequence of elementary steps of a radiation-chemical reaction. 

The kinetics of radical consumption with = 10 s can be studied at a flash duration of 1 ps. This allows the study of the kinetics of the bimolecular reaction with =10 s or = at a length of the reactor of 10 cm / = 0.1. Usually = l/(mol s), and constants up to lo" l/(mol s) are accessible for measurement. The method is widely used for measuring rate constants of atom and radical recombination in solution, reactions of molecules in the excited Uriplet state, electron transfer between radicals, and fast reactions of radicals with molecules (see Chapter 7). 

The thermal reversal of the photochemical a-cleavage, i.e., the direct recombination of the resulting radical pair or diradical, can be recognized as such only when at least one of the a-atoms is chiral and is epimerized in the process. In fact, the frequently rather low quantum yields observed in the phototransformations of nonconjugated steroidal ketones may be largely due to the reversal of a-cleavage. 

Similarly, we used the method of semiconductor sensors (SS) to study heterogeneous recombination of NH2 and NH radicals [2, 3], as well as hydrogen [4], oxygen [5], and nitrogen [6] atoms. 

Thus, we considered a number of examples of application of the sensor technique in experiments on heterogeneous recombination of active particles, pyrolysis and photolysis of chemical compounds in gas phase and on the surface of solids, such as oxides of metals and glasses. The above examples prove that, in a number of cases, compact detectors of free atoms and radicals allow one to reveal essential elements of the mechanisms of the processes under consideration. Moreover, this technique provides new experimental data, which cannot be obtained by other methods. Sensors can be used for investigations in both gas phase and adsorbed layers. This technique can also be used for studying several types of active particles. It allows one to determine specific features of distribution of the active particles along the reaction vessel. The above experiments demonstrate inhomogeneity of the reaction mixture for the specified processes and, consequently, inhomogeneity of the... 

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