Page 197 - Radiochemistry and nuclear chemistry

Radiation Effects on Matter 181

Table 7.4. Onsager radii, electron mobilities and free ion yields

Liquid r c mobility G(free ions)
(nm) (cm2/V s) (~mol/J)
Neopentane 32 55 0.09 - 0.11
Cyclohexane 28 0.35 0.016 - 0.02
Benzene 25 - 0.005 - 0.008
Methanol 2.3 - 0.2
Water 0.7 - 0.28

The distance rr (the Onsager radius) at which the potential energy of an ion pair
corresponds to the thermal energy k T is, according to Onsager,

r e = e 2/47rE 0~rkT (7.13)

where r and r is the permittivity of free space and in the medium at distance re,
respectively (the Onsager equation). The probability of an electron escaping its positive ion
to become a free ion is equal to e -r/r (r is the distance travelled by the electron before it
becomes thermalized) depends on the electron mobility in the medium. Most organic liquids
have a lower dielectric constant than that of water and the free ion yields are, therefore,
generally lower, see Table 7.4.
Electrons escaping their positive ions are gradually thermalizeA and solvated whereas
electrons that recombine with their geminate (i.e. original ion pair) radical cations form
excited molecules. The radical cations may react with solvent molecules. The types of
reactions to be expected are e.g. proton, hydrogen atom and hydride ion transfer. Excited
molecules are thus formed directly and by geminate ion recombination
A ~. A*

A .... ~A + +e-
A + + e- ~A**~A*

The highly excited (electronically and/or vibrationally) molecule A** formed by charge
recombination may (within 10-11s) lose part of its energy rapidly through collisions with
its neighboring molecules. The excited molecules can return to their ground state by several
processes. Singlet states (antiparallel spins of valence electrons) de-excite within 10-8 s by
fluorescence, whereas the phosphorescence of triplet states (parallel spins of the outer
electrons) requires 10 -5 to 10 s. Alternatively the excited molecules can undergo
unimolecular isomerization reactions or dissociate into highly reactive radicals.
Organic molecules in general have more atoms than water and, therefore, the formation
of a higher variety of products is to be expected. Both molecules smaller than the original
and polymeric products are formed.
The effects of radiation on organic molecules have been shown to be strongly dependent
on the molecular structure. Since the excitation energy is rapidly spread out over the whole
molecule, one would expect the weakest bond to rupture, producing two radicals, provided