64 REACTIONS INDUCED BY IONIZING RADIATION 



photoexcitation. Indeed, internal conversion appears to be Of dominant 

 importance in determining the course of the radiation chemistry of 

 complex molecules (17). 



Excited molecules may also be formed by the recombination of ion 

 pairs. Since the energy of ionization exceeds that required for the dis- 

 sociation of a chemical bond, it should be expected in most cases that 

 dissociation will follow the recombination of an ion pair. The behavior 

 of certain complex molecules (18), particularly aromatic hydrocarbons, 

 is an interesting exception to this general rule. A positive ion may be 

 neutralized by either a negative ion or an electron. There are probably 

 no important restrictions upon the recombination of positive and nega- 

 tive ions, in either condensed or gaseous systems. The capture by an 

 isolated simple molecule of an electron accompanied by the emission of 

 a photon is a very inefficient process (19). However, in gases even at 

 relatively low pressures (say, 50 mm), the system, ion and electron, is 

 on the average so coupled with neighboring molecules that recombination 

 to a highly excited state of the molecule should occur with a high yield 

 by what is effectively a triple collision. 



Ionization 



As has been discussed by the physics panel, molecules can be ionized 

 by impact (3) with charged particles such as alpha particles, beta par- 

 ticles, protons, and electrons. Molecules are also ionized by interaction 

 with high-energy photons, x-rays, or gamma rays. High-velocity neu- 

 trons also indirectly induce ionization. In addition to impact ionization, 

 a molecule containing high enough excitation energy may spontaneously 

 ionize by a process (20) analogous to predissociation. This phenomenon 

 is called preionization when it involves the valence electrons and the 

 Auger effect when it is in the x-ray region. Molecules may also be 

 ionized by thermal impact with an excited molecule or atom (for example, 

 2 ^*S He), provided the energy of excitation is greater than the energy of 

 ionization of the molecule concerned. Processes such as the combination 

 of two excited atoms (for example, 6 ^Pi Hg) to form an ionized molecule 

 (Hg2'^) and an electron have also been observed. 



A wide variety of ionized molecules can be produced from a single 

 compound (21) by electron impact if the energy of the electron is 

 sufficient. As has been shown by mass spectrographic studies, single 

 ionization of the original molecule is usually the most probable process 

 at reasonably low electron energies. However, multiple ionization, as 



