Spur reactions

The master equation methodology can be readily generalized to multiradical spurs, but it is not easy to include the reactions of reactive products (Green et al, 1989 Pimblott and Green, 1995). This approach is therefore limited to spur reactions where the reaction scheme is relatively simple. 

The sources of g(H2) according to deterministic [9] and stochastic [10] modelling are processes (I), (III), and (V) and also the spur reactions (R4) (R5) (R6). As shown in Table 1, the deterministic model indicates that approximately 50% of g(H2) is produced in reactions (R4) and (R5), and similar results are predicted by the stochastic model [10b,20]. The models also predict that the extent of reactions (R4) and (R5) increases with LET. For an increase in LET from 0.2 to 60 eV nm g(H2) increases to 0.27 and 0.42 molecules (100 eV) due to reactions (R4) and (R5), respectively [21]. These data, together with numerous experimental results, are not consistent with the conclusion [17] that H2 is generated mainly through reaction of with H2O. There will, of course, be no produced under conditions where e is scavenged efficiently in sufficiently concentrated solution. However, it does not follow that reactions (R4) and (R5) do not contribute to g(H2) in dilute solution or pure water. 

There seems to be little doubt from modelling [9,10,19,20] and experimental results (see Fig. 1) that the major precursor of the molecular yield of H2O2 is OH through reaction (R8) in Table 1. That reaction (R8) is a more important spur reaction than reactions (R4) and... 

To explain their results, Sutton et al. [56] suggested that spur reactions (R2) and (R8) in Table 1 be replaced by reactions (25) and (26), which are equally efficient ... 

Although it is possible to convert OH to e through reactions (70) and (30), the slowness of these reactions means that conditions of 100 atm of hydrogen and high pH are required. Nevertheless, these have been realized in experiments crucial to the measurement of spur reactions (R5) and (R6) in Table 1 [91,92] ... 

Usually, the average concentration of a reactive species such as OH or H is not directly observed in pulse radiolysis experiments, partly because the timescale of spur reactions is so short (< 10 ns) that most spur reactions have occurred during the radiation pulse which produces these species and partly because these species are very difficult to monitor on such a timescale. Instead, solutes are often added to water prior to radiolysis and the quantity of products formed by reaction of the solute... 

The first-order decays imply that the electrons recombine with some cations by the so-called spur reaction the electrons recombine always with their counterpart cations. The increase of the G value (see Fig. 3) shows that the added 2-methylpentene-l provides the sites for the stable electron trapping as well as stabilizing the positive charges in the glass. As far as the present authors know, the evidence for the cation radicals of 3-methylpentane has not been obtained by ESR. 

The key reactions of the radiolysis of water leading to the formation of OH, eaq and H and the conversion of eaq into a further OH have been discussed in Chapter 2.2. Here, it is sufficient to recall that the spur reactions are over in ca. 10-8 s, and from thereon the distribution of the radical species is practically homogeneous. 

Mogensen, O.E. (1974). Spur reaction model of positronium formation. J. 

Eq.(7) thus identifies with eq. (1), with k = aVn/(a [ + am + aiV). All empirical equations given before can be obtained in this way and extension to more complex cases (e.g., successive and competing spur reactions) is easily done (19). Developments of this approach are possible, such as by taking into account the statistical distribution of the solute molecules in the spurs. Although their physical grounds are of course questionable, the equations derived provide a quantitative approach to the data and the parameters (k, K, f, etc) prove to be very useful for comparisons (between solutes, as a function of temperature, of solvent to correlate with data from other fields, etc). The usefulness of such simple treatments may well reflect a genuinely simple situation for the actors of the spur processes where probabilistic factors may have a more important impact than detailed dynamics. 

In radiolysis, one of the most important reactions of solvated electrons is recombination with positive ions and radicals that are simultaneously produced in close proximity inside small volumes called spurs. These spurs are formed through further ionization and excitation of the solvent molecules. Thus, in competition with diffusion into the bulk, leading to a homogeneous solution, the solvated electron may react within the spurs. Geminate recombinations and spur reactions have been widely studied in water, both experimentally and theoretically, ° and also in a few other solvents. " Typically, recombinations occur on a timescale of tens to hundreds of picoseconds. In general, the primary cation undergoes a fast proton transfer reaction with a solvent molecule to produce the stable solvated proton and the free radical. Consequently, the... 

As is known, the estimation of G values of water decomposition products can be done by pulse radiolysis techniques or steady state radiolysis with product analysis methods. For pulse radiolysis, although a direct measurement of these transient species is desirable, it is difficult to be effectuated by nanosecond pulse radiolysis because of the acceleration of spur reactions and/or the limitation of detection techniques (e.g. the absorption of OH radical is in deep UV with a rather small absorption coefficient). One is forced to adopt the scavenging method, that is, to use a chemical additive to react with the transient species and form another easy-to-detect and relatively stable product. In this section, we mainly introduce the estimation of G values by pulse radiolysis, with the support by y-radiolysis of some aromatic compounds. 

The yields of the initial products of Eqs. 4-6 are not yet established unequivocally. The results of attempts to model the early stages of water radiolysis to match up with experimental measurements that are limited to about 10 ° s suggest that G(ions) and G(excitation) have values of about 0.5 and 0.1 pmol J , respectively [5]. Thus, around 40 % of the initial yields are consumed by the spur reactions. The spur reactions are listed in Table 1. 

Time (calculated) at which the formation of the monitored species is complete, lamp. Time at which spur reactions are complete. 

Although much valuable kinetic and mechanistic information has been obtained by pulse radiolysis with pulses of lengths in the nanosecond to microsecond time range since the technique was invented, it was natural to want to achieve even shorter time resolution, for a) the direct observation of the events taking place in the early stages of radiolysis (10 " -10 ° s), such as electron solvation, ion recombination, spur reactions, and so forth, and b) the measurement of very fast reaction rates (see below). 

For historical reasons the G values in reaction (6) are known as primary yields and it is estimated that about 40% ofthe n f a/yields (G°) produced in reactions (3) - (5) are consumed by the spur reactions, .e. G°(e q ) 0.5 pmol J The spur reactions are listed in Table 1. 

Figure 5 depicts the decay of the solvated electron due to spur reactions in two different solvents, water and tetrahydrofuran. In both liquids, the solvent relaxation is very fast (less than 1 ps), therefore, the absorption signals on the picosecond time scale are due to the fully solvated electron. As the dielectric constant of tetrahydrofuran is low (e = 7.6 compared to 80 for water), the electrostatic attraction is not screened by the solvent and geminate recombination between the solvated electron and the cation can occur over long separation distances in contrast to water. Moreover, the mobility of ej in THF is roughly three times higher than that in water. That explains why the decay ofthe solvated electron is more important in tetrahydrofuran compared to water [19]. 

The decay ofthe solvated electron due to spur reactions is faster in a slightly polar solvent like THF than in water which is a very polar solvent. 

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