Electrons thermalization

The secondary electron distribution is not yet thermal. In order to measure the time taken to attain a thermal distribution, a technique sensitive to the energy of the secondary electrons is required. Warman [13,14] and coworkers have elegantly developed and utilised the a.c microwave absorption method. This technique relies on the variation of the mobility of electrons with energy and/or number density to produce a change in the absorption of energy from an applied microwave field. 

At constant number density, electrons are detected since their mobility is generally 1000 times greater than any atomic or molecular ion of either charge sign and hence 

Compound Thermalisation time t (sec) P = 1 atm. Thermalisation time t (sec) P = 1 Torr  

For some systems both thermalisation and electron capture processes are in operation. In the case of oxygen, for example, it has been found possible to resolve [18] the thermalisation process - first order in gas pressure, from the 

One may safely assume that in 1 atmosphere of an irradiated molecular gas all primary species are created and thermalised within 50 nanoseconds. 

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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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