Radiolysis stages

Thus, the only way to study the early radiolysis stages is to study them theoretically, especially with mathematical simulation. The correctness of our theoretical conceptions is determined by how well all the consequences of the theory agree with the available experimental data... 

We are fully aware of the extensive review literature concerning the mechanism of primary processes in interaction of the ionizing radiation with matter.5,1, 20 25 However, the most recent of the cited reviews has been written more than a decade ago. The last ten years are marked by intensive development of experimental studies and by the appearance of new theoretical conceptions that change some of the traditional views on primary processes. In this review we discuss the modern ideas concerning the primary radiolysis stage that take into account the latest developments in this direction. 

Some of the aspects of the primary radiolysis stage have been recently discussed in Proceedings of NATO Advance Study Institute.26... 

Time Scale of Primary and Secondary Processes at Different Radiolysis Stages... 

Radiolysis stage Physical or chemical event Protection means Protection mechanism... 

On pulse radiolysis using CO f (—2.0 V) to reduee PCu(II)Ru(III) (pH 7, 20 °C) the behavior observed is in both cases very similar [50]. The concentration of C02 was adjusted so that there was <20% reduction of modified protein. As far as can be ascertained, reduction efficiencies in the first stage are about the same, with reaction partitioned between the Cu(II) (72%) and the... 

In this chapter, the various radiation chemical yields, known as G-values, are defined as follows g(X) is the yield of the species in reactions (6) and (7) G°(X) is the yield of the initial products of water radiolysis at the end of the physicochemical stage and G(X) is an experimentally measured yield. In some publications Gx, rather than g(X), is used to represent the primary yield of the species X. The yields in reactions (6) and (7) are numerically very similar, but one must be sure of the units that are being used to express them. In the older literature, it was common practice to quote G-values without units when the units were in fact molecules (100 eV) ... 

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) ... 

It would appear at this stage that a good deal of useful information has yet to be obtained by the pulse radiolysis method concerning the absorption spectra of the solvated electron in various organic liquids. Such data would help to remove uncertainties regarding the assignment of bands and would serve as criteria for the validity of developing models. 

Re-examination of the radiolysis of aqueous solutions of alanine (absence of oxygen) shows that electrons react rapidly with the cationic form, less rapidly with the zwitterion, and much less rapidly with the anionic form. These conclusions have been confirmed by pulse radiolysis. Rate constants for amino acids, peptides, proteins, and numerous other substances have been obtained. Critical evaluation of these and correlation with molecular properties is now well under way. In living systems the reactions of the hydrated electron vary with the part of the cell concerned, with the developmental stage of the cell, and possibly with the nature of any experimentally added substances. 

B. The Influence the State of Aggregation of a Medium Has on the Primary Stage of Radiolysis... 

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. 

The subsequently presented division of radiolysis processes into stages is different from the one originally proposed by Platzman.21... 

The previously described classification of elementary processes at different stages of radiolysis is presented in Table I. 

The subexcitation electrons lose their energy in small portions, which are spent on excitation of rovibrational states and in elastic collisions. In polar media there is an additional channel of energy losses, namely, the dipole relaxation of the medium. The rate with which the energy is lost in all these processes is several orders of magnitude smaller than the rate of ionizaton losses (see the estimates presented in Section II), so the thermalization of subexcitation electrons is a relatively slow process and lasts up to 10 13 s or more. By that time the fast chemical reactions, which may involve the slow electrons themselves (for example, the reactions with acceptors), are already in progress in the medium. For this reason, together with ions and excited molecules, the subexcitation electrons are active particles of the primary stage of radiolysis. 

The total yield of ions G, formed in a liquid at the primary stage of radiolysis cannot be measured experimentally. So the value of G, is either taken to be equal to the yield of ions produced by radiation in a gaseous medium, or, like rQ, is considered as a parameter whose value is chosen so as to get the best correspondence with the experimental dependence of Gfi on %. 

An opposite result has been obtained in Ref. 14, where, using the Monte Carlo method, the authors made a comparative study of the influence the state of aggregation of water has on yields of ionized and excited states and on the way electron energy is absorbed. According to Ref. 14, with transition from vapor to liquid, the number of ionizations induced by a fast electron increases, while the value of Wt (at Ee = lOkeV) lowers from 30.0 to 24.6 eV. Both in liquid water and in water vapor the yield of ions exceeds that of excited molecules. In our study,143 which we have made in collaboration with Sukhonosov, we simulated the primary stage of water radiolysis and have obtained a somewhat different result. 

At present we have performed calculations with the scheme of primary stage of radiolysis which differs from that in Ref. 16. 

As we have already discussed in Section VIII, at the physical stage of radiolysis the primary active particles (ions, excited molecules, and electrons) are localized in separate microregions—in the track structures. The dimensions of track structures, the concentration of active particles in them, and the subsequent transformations of these particles depend on the density of the medium. 

However, the spatial inhomogeneity in the distribution of reagents is not the only reason why the radiolysis of substances in the condensed state is different from that of gases. As we have already mentioned in Section VIII, as we pass from the gaseous state to the condensed one, at the primary stage of radiolysis we already observe a redistribution of yields of primary active particles (resulting in the increase of the yield of ionized states). Also different are the subsequent relaxation processes, as well as the processes of decay of excited and ionized states.354 Another specific feature of processes in a condensed medium is the cage effect, which slows down the decay of a molecule into radicals.355 Finally, the formation of solvated electrons is also a characteristic feature of radiation-chemical processes in liquids.356... 

Egusa et al. disclosed the behaviors of cationic intermediates in the initiation stage of the polymerization of styrene and a-methylstyrne by low temperature pulse radiolysis [24, 25], Figure 3 shows the transient absorption spectra for styrene in a mixture of isopentane and n-butyl chloride (4 1 by volume) irradiated with 4 /is pulses at — 165 °C. M, and M2 refer the absorption bands... 

---