190 C. REID 



polymeric material. What such a process does not provide in its simple form is 

 a mechanism for the stabilization of any of these polymers. Bonds are being 

 continuously broken as well as formed, and unless the breaking process can be 

 stopped or radicals stable against dissociation are formed, the equihbriimi mole- 

 cular weight must be expected to be rather low. 



The stabilization mechanism thus appears as the key step which may have 

 determined the mode of subsequent evolution, determining what kind of mole- 

 cule may be expected to 'grow*. 



Dissociation Prevented 



In polyatomic systems, energy in excess of that required for bond breaking 

 may be present for a finite time before the dissociation actually occurs. This 

 time increases rapidly as the number of vibrational modes in which the energy 

 may be accommodated is increased. If no means of disposing of the excess energy 

 is available, dissociation must ultimately occur, but if some means of energy 

 dissipation within the dissociation time is available, disruption will be pre- 

 vented. Probably speaking this dissipation may occur in two ways : 



(i) The molecule is fluorescent, rapidly re-emitting absorbed light and thus 

 surviving until attacked chemically and increasing its molecular weight. To 

 continue to invoke this process, as the molecule grows larger, means that where- 

 ever in the molecule light is absorbed, the excitation energy must be trans- 

 mitted to a luminescent group. Since we know that there are many more non- 

 luminescent than luminescent groups among biologically important molecules, 

 we arrive at the idea of a trap mechanism within the molecule, followed by a 

 radiative process. 



(2) The molecule is exceedingly well coupled to its environment, so that 

 absorbed energy can be lost before dissociation. This is a well-known pheno- 

 menon in crystals — including molecular crystals — but again some molecule in 

 the crystaUine aggregate probably will be decomposed unless a 'trap' is provided. 

 Dissipation as thermal energy within a crystal is also possible, though in mole- 

 cular crystals this is usually too slow to prevent dissociation. A similar pheno- 

 menon may also occur for the case of a single molecule strongly absorbed on a 

 suitable inorganic crystal, and such a heterogeneous system must be considered 

 as a possibility. 



Decomposition Occurs, followed by Recombination 



This kind of mechanism is particularly suited to the condensed state and has 

 been postulated to explain the results of many irradiation experiments. Two 

 possibihties appear: 



(a) Ions or radicals are formed, but are unable to move away from each other 

 because of Coulomb attraction, strong absorption, or bulky neighbours (cage 

 effect). 



(b) Very long-living radicals are formed which can move apart, but which do 

 not decompose into smaller fragments. 



In both these cases excess energy may be dissipated at leisure as vibrational 



