ENERGY EXCHANGE IN PHOTOREACTIONS 29 



of coordinates, which can then be treated independently (Curtiss and 

 associates, 1950, 1952). 



Returning to Zener's work, we find that his calculation for the efficiency 

 of transfer of the large vibrational quantum of the nitrogen molecule from 

 one molecule to another gives the high value of 10~^ which is compatible 

 with observation. This result is due to the participation of just two 

 internal degrees of freedom, in turn the result of the exact balance of 

 energy available against energy that can be taken up by the second mole- 

 cule. This is the condition for maximum efficiency and is called the 

 "resonance condition" or "resonance requirement." Its explanation is 

 especially clear in Zener's theory: the fewer pi's involved, the better; 

 the less the energy that must be added or taken from a third degree of 

 freedom, the greater the probability of transfer. Franck first proposed 

 this explanation. 



Qualitatively we can understand that nitrogen should be efficient in 

 any transfer process despite its normally low chemical reactivity. The 

 more strongly a bond oscillates, the more the atoms take on the proper- 

 ties of free atoms and so tend to combine more strongly with other atoms 

 that come within range. The result is that a rapidly vibrating oscillator, 

 when it strikes a second oscillator, will cause the second one to expand 

 and absorb energy. This makes for rapid communication of energy 

 between high-frequency oscillators, however they collide. Molecules 

 like water can be very efficient in transfer reactions despite their lower 

 vibration frequencies because they possess chemical affinity for many 

 substances. Thus nitrogen causes the collision path to climb high on 

 a high-potential-energy barrier, whereas water reduces the height of 

 the barrier, making extreme interpenetration of the colliding partners 

 unnecessary. In addition, the vibrational quanta of water are inter- 

 mediate in energy between the high-energy quanta of oxygen or nitrogen 

 and the small quanta of translation and rotation. 



Further information about energy-transfer processes in collisions on 

 single potential-energy surfaces has been secured from the effectiveness 

 of substances as third bodies in triple collisions leading to simple atomic 

 combination. A third body is necessary in such cases to remove the 

 extra energy so as to stabilize molecule formation. The rates of uni- 

 molecular reactions, the transfer of heat, and the study of shock-wave 

 propagation have also provided information. 



Energies of the order of 100 kcal/mole are produced in atomic combi- 

 nation, but only a fraction of this need be removed by a third body to 

 stabilize the formation of diatomic molecules. Originally this energy is 

 stored as high vibrational amplitude in the collision complex. Such 

 amplitudes favor penetration of higher potential surfaces with a conse- 

 quent increase in the probability of finding a suitable gateway. It has 

 been well established that chemical affinity is of prime importance in 



