The physical stage

Other predictions were that excited states produced in gases which are optically connected to the ground state would be found to be produced more rapidly than those for which optical transitions to the ground state are forbidden. In argon for example the production of optically forbidden states such as the Ap states (2p in Paschen notation) and the two metastable 3p 4s states (1 3 and I55) was predicted [3,5] to be delayed relative to the production of ions. 

These observations are qualitatively in agreement with the predictions from the TDSF theory however, a direct comparison between theory and experiment has not yet been made for the heavier rare gases. This delay prediction was validated by the observations by Burgers [7] in irradiated neon, where the production of neutral excited states were delayed with respect to the formation of excited ions. Cooper and Sauer [9] used a picosecond pulse radiolysis technique to investigate the formation and decay kinetics of several Ip states in the four rare gases neon, argon, krypton and xenon at low gas pressures ( 5 Torr). They found that all these states had delayed formation. 

At this point in the sequence of radiolytic events, we have a plasma containing hot , i.e. 5-20 eV, electrons. Prior to ion recombination or electron capture processes the energy needs to be further lowered to approximately thermal values. 

The weak dependence of the rate of formation of the emission on the pressure of the bulk gas was attributed to a collisional momentum transfer process between the electron and the monatomic rare gas, i.e. 

The data in Table 2 shows the subexcitation level data for five rare gases. Essentially, distributions of subexcitation electrons with a maximum energy of 19.8 eV in helium down to 8.6 eV in xenon can be generated. This gives an opportimity to generate and study plasmas with a range of electron energy. 

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At the end of the physical stage, which is within about 10 sec of the passage of the ionizing particle through the liquid, the track made by the particle contains H20", subexcitation electrons e , and electronically excited water molecules H2O in small clusters called spurs. From about 10 to 10 sec, the following processes are thought to occur and comprise the physicochemical stage [9,10] ... 

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. 

The physical stage, consisting of the absorption of the radiant energy by the irradiated system. Its duration is of the order of 10 s. 

Besides the preliminary stability study, regular checks should be carried out over the entire lifetime of the material. In some rare situations, over the certification projects conducted under the BCR activities, examples of instability of substances was demonstrated at certain storage temperatures. Figures 4.12a-b on organochlorine pesticides in animal feed (confirmed in fish oil) and 4.13a-b for polycyclic aromatic hydrocarbons illustrate such examples. For p,p -TDE the increase in content is generated by the decomposition of p,p -DDT. A similar effect is noticed for o,p -DDT which fully disappears over a period of six months at +40 C [12,14]. For benzo(a)pyrene it could not be demonstrated whether the decrease in content noticed was due to a decomposition of the PAH or to a decrease in its extractability due to the change of the physical stage in the matrix [46,47]. These materials looked stable at lower temperatures usually already at +20°C. Consequently they are stored at temperatures of +4 C or less. 

The objective of these studies has been to develop a reaction sequence model for flexible urethane foam which would account for the physical stages in the foaming process evident from foam rise measurements. In Figure 18, a generalized rise profile and gel profile" is used to summarize the reaction sequence model. 

A survey is given of the theory of the physical stage of radiolysis. Using the optical approximation to cross sections for the interaction between fast electrons and molecules, expressions have been derived for the yield g° of primary optical activations, and for the total absorbed energy Qtot It is shown that the total yield g of primary activations is conveniently discussed as a sum g° + gs, where the first term includes the action of fast electrons, while g8 describes the action of slow electrons (kinetic energy less than about 100 e.v.) on molecules of the medium. This approach is compared with Platzmans considerations on primary yields and the differences are pointed out. Finally, theoretical results of the present approach are applied to the analysis of the initial structure of the track of a fast electron, consisting of spurs, blobs, and short tracks. 

The most characteristic type of primary activations are the electronic transitions of molecules which are much faster than other response of the irradiated medium. This enables one to consider separately the physical stage of radiolysis, at the end of which a certain ensemble of excited and ionized molecules is formed in the medium. Each of the activated molecules possesses a particular amount of energy available for subsequent processes. The initial distribution and yields of individual primary activations are dealt with by the theory of primary radiation chemical yield (PRCY). We have studied the application of this theory to the radiolysis of gases in detail during the last years (16, 17, 18, 19, 20). Thus, in the formal expression—see (5), for the yield G(X) = %ngncompetitive reaction ways and remain much more obscure at the present. 

The physical stage of a (3- or y-radiolysis of a sufficiently diluted medium is described in this theory (see Table I) as the interaction of the degradation spectrum of electrons with the ensemble of isolated molecules of the gas. The probabilities of individual inelastic collisions are characterized by their cross sections and the solution of the problem is,... 

This formula combines optical data such as df/dE and M2ion, which is an integral over all ionized states analogous to M2 in Equation 6, with empirical radiation-chemical data on W. Equation 9 is essentially of the form g = const X g° and thus represents an alternative to, and should be compared with our view, g = g° gs. It does, in fact, extend the optical approximation expressed by Equation 1 to the totality of primary activations in the physical stage and yields, in particular, the approximative conclusion y = y°. 

Without going into details to be discussed elsewhere (20), we may point out the main difference between the two approaches as follows While our approach tends to emphasize the contribution gs of slow secondary electrons to the primary yields, Platzman s treatment puts generally more emphasis on the internal energy of the ionized species than we do. Consequently, his absolute values of primary yields are, in general, lower than ours, and a significant part of the observed decomposition is implicitly expected to occur in subsequent physicochemical and chemical stages of radiolysis. In contrast, our approach explains most of the observed decomposition by the primary processes of the physical stage. 

In addition, corrosion performance depends on the physical stage of the chemical (solid, liquid, or gas), the concentration, temperature, and the presence of trace amounts of water or other iinpurities. Consider the role of trace amounts of water versus the anhydrous chemical. In some chemicals, such as phenol, a trace amount of water (0.1%) will decrease corrosion (Fig. 14), while trace amounts of water in liquid sulfur dioxide (SO2) will form sulfuric acid and promote corrosion. Temperature is also important. Laboratory tests showed that 3003 was compatible with phenol up to a terrqrerature of 50 C (120 °F) but... 

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