Electrons thermalized

Reaction type Number of electrons Thermally allowed Photochemically allowed... 

Most radiation-chemical reactions are thermal in nature those considered in the diffusion-kinetic scheme are essentially thermal reactions (see Chapter 7). In polar media, electron thermalization is presumed to occur before solvation (Mozumder, 1988). However, ionization processes usually involve transfer of energy in excess of the ionization potential (see Chapter 4). Therefore, mechanisms of thermalization are important for radiation-chemical effects. 

On the other hand, electron thermalization, although fast on the scale of thermal reactions, can still be discerned experimentally. In the gas phase, it exhibits itself through the evolution of electron energy via time-dependent reaction rates. In the liquid phase, the thermalization distance in the field of the positive ion is the all-important quantity that determines the probability of free-ion generation (see Chapter 9). In this chapter, we will deal exclusively with electron thermalization. 

Shizgal et al. (1989) have listed a large number of processes that require an understanding of electron thermalization in the gas phase. These range from radiation physics and chemistry to radiation biology, and connect such diverse fields as electron transport, laser systems, nuclear fusion, and plasma chemistry. Certainly, this list is not exhaustive. 

In a nonattaching gas electron, thermalization occurs via vibrational, rotational, and elastic collisions. In attaching media, competitive scavenging occurs, sometimes accompanied by attachment-detachment equilibrium. In the gas phase, thermalization time is more significant than thermalization distance because of relatively large travel distances, thermalized electrons can be assumed to be homogeneously distributed. The experiments we review can be classified into four categories (1) microwave methods, (2) use of probes, (3) transient conductivity, and (4) recombination luminescence. Further microwave methods can be subdivided into four types (1) cross modulation, (2) resonance frequency shift, (3) absorption, and (4) cavity technique for collision frequency. 

TABLE 8.1 Experimental Electron Thermalization Times in Various Gases at -300K... 

TABLE 8.2 Relative Importance of Various Processes to Electron Thermalization in H2... 

FIGURE 8.3 Geometric and energetic relationship for electron thermalization by random walk in liquid hexane in the presence of the geminate positive ion. Here = fid). Reproduced from Mozumder and Magee (1967), with the permission of Am. Inst. Phys. . 

FIGURE 8.4 Electron thermalization distance distribution in n-hexane at 290K starting from an initial separation 23A. See text for details. Reproduced from Rassolov (1991). 

Mozumder s (1988) conjecture on electron thermalization, trapping and solvation time scales in liquid water is based on combining the following theoretical and experimental information ... 

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