62 REACTIONS INDUCED BY IONIZING RADIATION 



is thermally activated; that is, it is a "hot" molecule. As such it can 

 undergo chemical changes typical of thermal reactions. It may dis- 

 sociate into two radicals or into two stable molecules. Decarboxylation 

 is a reaction of the latter type. In giant molecules, such as proteins, it 

 is probable that the oscillational energy would not be evenly distributed 

 over all the degrees of freedom of the molecule, but would for a time be 

 confined to one segment of the molecule. Although energy which is 

 distributed over many degrees of freedom cannot break ordinary chemical 

 bonds, it could be sufficient to break weak linkages such as hydrogen 

 bonds. In this way internal conversion might easily be responsible for 

 reversible denaturation of proteins. 



The fate of an excited molecule can be profoundly influenced by its 

 environment. A simple molecule, such as hydrogen iodide, when dis- 

 solved in a chemically inert solvent can be photochemically dissociated 

 just as it is in the gas phase. However, the resultant atoms will be caged 

 in by surrounding solvent molecules. Before they can escape from this 

 cage they will undergo many collisions with one another. As a result 

 there is a considerable probability that they will recombine before they 

 can separate. This F ranch- Rabi now itch (12) effect can be responsible 

 for a noticeable decrease in the quantum yield of a dissociation process 

 occurring in a solution. The probability of escape from the cage is 

 greater if the atoms are small and if they are initially endowed with high 

 kinetic energy. 



When an excited complex molecule in a solution undergoes an act of 

 internal conversion, the probability of its losing its oscillational energy 

 is greatly increased, since it is constantly in a state of multiple collision 

 with the solvent molecules. Should the excited complex molecule be 

 dissociated, its radicals will be hemmed in by the solvent cage. The 

 recombination of complex radicals frequently requires some energy of 

 activation. Furthermore, complex radicals must be in a definite orienta- 

 tion relative to one another before they can recombine. These steric and 

 energetic requirements greatly reduce the rate of recombination of 

 complex radicals and probably more than counterbalance their decreased 

 rate of escape due to their size. It should be expected, therefore, that 

 the cage effect will be less efficient in preventing the separation of 

 complex radicals than of atoms or simple radicals. The dominant factor 

 influencing the decomposition of excited complex molecules in a con- 

 densed system is most likely the rapid removal of their oscillational 

 energy after the act of internal conversion. 



Excitation energy may migrate through many molecules in a crystal 

 in which the binding forces are strong and in which a very good resonance 

 exists between the neighboring fundamental units of the crystal. This 



