Time Scale of Events

In Table 3 is shown the time scale in which some of the processes which have been discussed take place. These times are necessarily approximate and depend on the substrate. In some cases the time scale for different processes will overlap. 

10- Passage of ionising radiation with subsequent ionisations. 

10 Vibrationally excited species appear. Ion-molecule reactions. Dissociation of electronically excited species to give radicals. 

1 Reactions essentially complete except for post-irradiation phenomena in some systems. 

The initiation of chemical reactions by low energy radiation, e.g. ultraviolet light, and by high energy ionising radiation, is compared in Table 4. 

TABLE 2.1 Approximate Time Scale of Events in Radiation Chemistry 

Generally speaking, the species that exist at the end of one stage serve as the input of the next stage. In this sense the qualification early is relative. In many 

Chapter 2 Interaction of Radiation with Matter Energy Transfer 

FIGURE 2.1 Distribution of deposited energy among different degrees of freedom as a function of time, represented by pt = -log t (sec). Note that, for exothermic reactions, low-grade heat can terminate above the level of absorbed energy. Luminescence can only alter the picture in a minor way. Reproduced from Mozumder (1969a), by permission of John Wiley Sons, Inc.  

Therefore, a reaction needs to proceed within that certain period of time to make the products detectable in the mass spectrum, and for this purpose there is a need for some excess energy in the transition state. 

Mass analyzer Flight path fml Acceleration voltage fVl Typical m/z Flight time [ps] 

The terminology for ions has been coined as a direct consequence of the classical mass spectrometric time scale. Nondecomposing molecular ions and molecular ions decomposing at rates below about 10 s will reach the detector without fragmentation and are therefore termed stable ions. Consequently, ions dissocial- 

There is no justification for such a classification of ion stabilities outside the mass spectrometer because almost all ions created under the conditions inside a mass spectrometer would spontaneously react in the atmosphere or in solvents. Nevertheless, this classification is useful as far as ions isolated in the gas phase are concerned and is valid independently of the type of mass analyzer or ionization method employed. 

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Table 1 Approximate Time Scale of Events in Radiation Chemistry, for Example, of Liquid Water...

The approximate time scale of events initiated by the absorption of energy by water from the incident ionizing radiation is shown in the following scheme ... 

Cool flames are difficult subjects for quantitative study since the time scale of events is generally too short to allow the use of conventional sampling. In addition, their non-isothermal character (which implies rate coefficients which change as reaction progresses) makes it difficult to develop theoretical models which satisfactorily describe the more important features (the periodicity and temperature rise). It is outside the scope of this review to discuss the more general theoretical aspects of cool-flame phenomena, and the reader is referred to VoL 2, Chap. 2 of this Series and also to the work of Yang and Gray [113], Halstead et al. [114, 115] and others [112,116,117]. 

Three different stages, physical, physico-chemical and chemical, can be distinguished after absorption of an electron (or any other ionizing radiation) by matter. The approximate time scale of events according to Mozumder [10] is given in Table 4. The time needed by a fast electron to traverse a molecule is calculated on the assumption of an electron with a... 

APPROXIMATE TIME SCALE OF EVENTS IN RADIATION CHEMISTRY [10]... 

Let us now consider in detail the radiolysis of ethyl chloride. Although it is well known that the relative intensities of ionic fragments observed in the low pressure mass spectrum of a compound do not necessarily represent the abundances of these ions in a radiolysis system at higher pressures (owing to a difference in the time scale of events), such a spectrum still provides a useful indication of possible primary ionic dissociation modes. The primary ions observed in the mass spectrum of ethyl chloride and their relative abundances are shown below. 

Figure 2 compares the model predictions with data from mice for each of the body regions, and excellent agreement is obtained. Figure 3 shows clinical plasma data in man. Although the time scale of events is quite different, the same pharmacokinetic model successfully predicts them. 

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