10 



RADIATION BIOLOGY 



ment of a mass particle, which is originally given the proper total energy, 

 approximates'^ the changes of geometry in time for systems of a few 

 atoms. The movie record of such a process as, for instance, the oscil- 

 lation of a ball in the potential well of the stable triatomic molecule 

 would yield the approximate history of energy transfer for the process 

 in question. We shall make use of this property. 



The potential-energy diagrams thus far considered have been for 

 chemically stable systems (for example. Fig. 1-4). The method for their 

 calculation, as previously outlined, applies equally well to unstable sys- 

 tems and has, indeed, received its principal development in this appli- 

 cation. The region of the surfaces of these diagrams corresponding to 



A B + C 



A—B DISTANCE 



Fig. 1-5. Typical contour map of potential energy for the simple replacement reac- 

 tion A + BC —* AB + C. The dotted line is a collision path in which translational 

 energy is converted into vibrational energy. 



small distances between the stable entities involved is now characterized 

 by a potential barrier rather than a well. Figure 1-5 is an example of 

 such a system. The dotted path of a rolling ball describes the redis- 

 tribution of energy that takes place between the colliding molecule and 

 the diatomic species BC in a single collision. An inelastic collision takes 

 place, in that part of the relative kinetic energy of translation of the two 

 molecules is transferred to vibrational energy of BC, as indicated by the 

 increase in oscillatory motion of the system across the potential trough. 

 Had the original translational energy been high, the system would have 

 crossed the potential barrier to produce a simple displacement reaction 

 (the figure represents coUision along the longitudinal axis of the diatomic 

 molecule only). For present purposes the energy is taken as less than 

 the value necessary to produce reaction. In contrast to the situation 

 for stable entities, systems containing complex molecules are not so 



^ Actually, if the vertical motion is kept small by an appropriate scaling factor, this 

 approximation can be made excellent. 



