The chemical stage

A range of techniques are available for use in pulse radiolysis experiments. 

They are fully presented in several texts, see for example the compilation of Baxendale and Busi [15]. To summarise, the detection techniques used in this field are mainly D.C. and time resolved microwave conductivity (TRMC), and Optical Absorption and Emission Spectroscopy. 

The passage of microwaves through a weakly ionised medium is influenced by the number, and type, of charges present. Anything which alters 

In most experimental studies so far reported in gases, electrons are detected since their mobility is generally 1000 times greater than an atomic or molecular ion of either charge sign and hence 

This relationship may be used to define two useful limiting experimental conditions  

---

In theory, one assumes the formation of radicals before the chemical stage begins (see Sect. 2.2.3). These radicals interact with each other to give molecular products, or they may diffuse away to be picked up by a scavenger in a homogeneous reaction to give radical yields. The overlap of the reactive radicals is more on the track of a high-LET particle. Therefore, the molecular yields should increase and the radical yields should decrease with LET. This trend is often observed, and it lends support to the diffusion-kinetic model of radiation-chemical reactions. 

Table 1 Reaction Scheme of the Chemical Stage in Water Radiolysis at 298 K for Low LET Radiation...

The time dependence of the yield of hydrated electrons during the chemical stage of water radiolysis (ca. 10 to 10 sec) can be obtained from scavenging studies and by direct observation using pulse radiolysis. In the former case, the scavenger dependence G(S) is converted into the time dependence using the inverse Laplace transform of F(S) to F(t), which is given by Eqs. (19) and (20) ... 

A typical plot of the change in DOC concentration and biodegradable fraction as a function of the specific ozone dose shows a continuous increase in the biodegradable fraction until a certain maximum is reached. Further ozonation decreases the biodegradable fraction (Figure 6-2). This is an optimization problem the compounds must be made bioavailable, but mineralization in the chemical stage must be minimized. 

The most rapid bimolecular reactions must be the ion-molecular ones. Their duration can be limited only by the time of collision, thus being 10 13-10 12 s. The recombination time of radicals that have escaped from the cage depends on their concentration in the track. For close pairs of radicals the recombination may already begin in 10 us. From this moment on we can consider the chemical stage of radiolysis to have begun. 

In condensed systems (liquids and solids), the main reactive species produced in the physiochemical stage that react in the chemical stage are free radicals. Their primary modes of reaction are atomic abstraction, radical recombination, and addition to 7i-bonds. 

The study of ionic states of aromatic molecules has dealt largely with the radical anions of these molecules in polar liquids, and more recently to a lesser extent with aromatic cations. The study of electronically excited states has been concerned principally with the triplet state in both non-polar and polar liquids, and to a lesser extent with the singlet state. The direct observation of these reactive species has provided some understanding of fundamental phenomena in radiation chemistry such as the extent of charge separation in polar liquids and the persistence of this charge separation into the chemical stage of events, the mode of formation and yield of both ionic and electronically excited... 

The usual extrapolation of experimental knowledge back along the time sequence of radiolytic processes has provided us with the picture of a radical track which is formed at the beginning of the chemical stage. While the further development of this track by diffusion and chemical reactions had been successfully treated in great detail by the theory of diffusion kinetics, the initial structure of the track was pictured only... 

The time dependence of the yield of hydrated electrons during the chemical stage of water... 

The optimal conditions for synthesis of different kinds of final products have been found. The chemical stages and forming mechanism for a number of nitrides have been found as well. The technological process of refractory powders synthesis in SHS-Az regime has been developed with the use of complexing halides of ammonia and elements to be nitrided. 

The ultimate reaction in the anodic formation of CI2 involves the transfer of two electrons from two Cl ions. Therefore, after the formation of Cl(+1) (i.e. after Cl - 2e Cl(+1) it should react with a Cl" ion, resulting in the formation of CI2. This chemical reaction (or sequence of reactions) cannot be slow, since otherwise the rate of the reverse process, i.e. the decomposition of CI2 into Cl and Cl(+1), which would limit the cathodic reduction of Cl2> would not depend on the chloride concentration. Experimentally, we observe an order -1 with respect to chloride for the cathodic reaction. This unambiguously indicates that the chemical stage is reversible (an increase in the chloride concentration decreases the equilibrium concentration of Cl(+1)). 

---