238 RADIATION BIOLOGY 



doxical fact that the mobihty of He^ ions in heUum is greater than that of 

 He+. 



It should be mentioned that, in "fast" coUisions at any rate, even 

 identical molecules are not in exact resonance, because the equilibrium 

 nuclear separations of ionized and neutral molecules are in general differ- 

 ent. On the other hand, possible small changes in oscillation energy, 

 which would not violate the Franck-Condon principle in "slow" colli- 

 sions, might tend to improve resonance in the impact of a dissimilar ion 

 and molecule. Such molecular effects do not appear to have been studied. 



In a very large polyatomic molecule, "charge exchange" may occur 

 internally, the electron vacancy migrating until a more stable configura- 

 tion is reached. This process could well be of importance in radiation- 

 chemical and radiobiological reactions. It is, of course, a mechanism dis- 

 tinct from the electron migration discussed in Sect. 3-4d. 



In conclusion, mention must be made of the fact that other elementary 

 processes, such as exchange reactions, may also involve a transfer of 

 charge. An example is 



H2 + Br+ -> HBr + HBr+ 



4-3b. Dissociation. Ionization of a molecule, either by light absorption 

 or by electron impact, usually leaves the ion in an excited vibrational 

 state (Sect. 4-1), and the ion will dissociate (monomolecularly) if its dis- 

 sociation energy is smaller than this vibrational excitation energy. Such 

 dissociation is of very common occurrence. (In homopolar diatomic mol- 

 ecules the vibrational excitation energy is often smaller than the dissocia- 

 tion energy, and they may be ionized without permanent chemical effect.) 

 The time required for dissociation to take place is strongly influenced by 

 the complexity of the molecule. In general, diatomic molecules dis- 

 sociate at once, i.e., in a time of order of magnitude 10~^^ second; poly- 

 atomic molecules, in which the vibrational energy can fluctuate among 

 many degrees of freedom, may not dissociate until after a much longer 

 time (cf. Sect. 3-3c) — as long as 10"^ second in some cases. ^^ 



16 Recent techniques of mass spectrometry have made possible a determination of 

 lifetime for dissociation of certain polyatomic ions (cf., for example, Hippie, 1948). 

 The measurements are restricted to ions that have a lifetime of order of magnitude 

 10-6 second; an example is C4H+ -^ C3H+ + CH3. Apparently the reason for this 

 comparatively long lifetime has not been conclusively established. Two evident 

 possibilities are: (1) that the ion is in a high vibrational level of the ground electronic 

 state, and the measured lifetime reflects the time required for this vibrational energy 

 to be concentrated in a particular degree of freedom; and (2) that the ion is in an 

 excited electronic state that is metastable, it therefore cannot radiate in a period of 

 10"'' second, and its lifetime stems from the time required for internal conversion to 

 the ground state, in which the high vibrational energy soon dissociates the molecule. 

 In view of the fact that IQ-^ second is a relatively long time for a highly (vibrationally) 

 excited molecule to require for dissociation, explanation (2) would appear the more 

 likely. 



