Subexcitation

The molecular time scale may be taken to start at 10 14 s following energy absorption (see Sect. 2.2.3). At this time, H atoms begin to vibrate and most OH in water radiolysis is formed through the ion-molecule reaction H20+ + H20 H30+ + OH. Dissociation of excited and superexcited states, including delayed ionization, also should occur in this time scale. The subexcitation electron has not yet thermalized, but it should have established a quasi-stationary spectrum its mean energy is expected to be around a few tenths of an eV. 

In liquefied rare gases (LRG) the ejected electron has a long thermalization distance, because the subexcitation electrons can only be thermalized by elastic collisions, a very inefficient process predicated by the small mass ratio of the electron to that of the rare gas atom. Thus, even at a minimum of LET (for a -1-MeV electron), the thermalization distance exceeds the interionization distance on the track, determined by the LET and the W value, by an order of magnitude or more (Mozumder, 1995). Therefore, isolated spurs are never seen in LRG, and even at the minimum LET the track model is better described with a cylindrical symmetry. This matter is of great consequence to the theoretical understanding of free-ion yields in LRG (see Sect. 9.6). 

Frohlich and Platzman (1953) developed a detailed electromagnetic theory for the rate of energy loss of a subexcitation electron in a polar medium due to dielectric loss. Their final result may be expressed as... 

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] ... 

C. The Theory of Retardation of Subexcitation Electrons in Condensed Media... 

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. 

As a rule, the lowest excitation level in molecules is a triplet level. Since such states are efficiently excited by slow electrons (see Section IV.B.3), it is the energy of the lowest triplet level that one should take as a boundary energy for subexcitation electrons rather than the energy of the first singlet excitation level, as was done in Ref. 23. 

The role of subexcitation electrons is most important when the irradiated medium contains small amounts of impurity molecules the excitation energy ha) 0j (or the ionization potential I ) of which is below h(o0l. Such additive molecules can be excited or ionized by the subexcitation electrons the energy of which is between h(o 0j and fuom, and, consequently, the relative fraction of energy absorbed by an additive will be different from what it should be if the distribution of absorbed energy were solely determined by the relative fraction of valence electrons of each component of the mixture.213 214 According to estimates of Ref. 215, this effect is observed when the molar concentration of the additive is of the order of 0.1%. This selective absorption with ionization of additives has been first pointed out by Platzman as an explanation for the increase in the total ionization produced by alpha particles in helium after small amounts of Ar, C02, Kr, or Xe were added (the so-called Jesse effect).216... 

The subexcitation electrons are characterized by a certain distribution function 17(E) defined in the range 00l. The first speculations concerning the form of t](E) where made by Magee and Burton,217 who analyzed the energy distribution of electrons ejected during photoionization of atoms. Later, Platzman has proposed the following general form... 

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