ENERGY EXCHANGE IN PHOTOREACTIONS 



25 



and its availability will largely be functions of the chemical affinity of 

 the collision partners. With the exception of ions of like sign, all pairs 

 of molecules will have at least some positive minimum of mutual affinity. 

 When this minimum is very low, as, for example, it is when one collision 

 participant is a rare-gas atom, distortion of the equipotential lines will 

 occur only at very close distances of approach and hence at high values 

 of potential energy. The lower-lying equipotential lines will be sym- 

 metrical parabolas with parallel asymptotes. These parabolas tend to 

 reflect the trajectory without change in vibrational quantum number. 



Table 1-1. Effect of Various Gases on Dispersion of Sound in 



Other G.\ses 



The rare gases are consequently poor agents for the establishment of 

 thermal equilibrium in internal degrees of freedom. Helium is superior 

 to the others in this respect, as determined by its effectiveness in pre- 

 venting the dispersion of sound (Table 1-1). This behavior may be 

 attributed to the small size of the atom which favors close approach and 

 high relative translational velocities. High velocities drive the trajecto- 

 ries onto the higher regions of the potential surface in which the gate- 

 ways are located. 



Studies of the dispersion of sound have provided a major part of the 

 information on collisional exchange of low-energy quanta. Sound in 

 liquid or gas phases is transported in compression waves, which produce 

 alternate high and low pressures in a given small region. Since the proc- 

 ess is essentially adiabatic in a thermodynamic sense, the temperature 



