20 RADIATION BIOLOGY 



energy-transfer mechanisms. For example, in most species of plants and 

 algae, chlorophyll molecules are always associated closely with carotenoid 

 pigments such as /3-carotene (Fig. 1-9) (Rabinowitch, 1945, p. 412). It 

 seems entirely possible that the carotenoid acts as a power line between 

 chlorophyll molecules to increase the efficiency of light use. 



3. ENERGY TRANSFER DURING ADIABATIC COLLISIONS 



3-1. ABSOLUTE THEORY OF COLLISIONS 



Most energy-transfer processes take place during the close approach, 

 or collision, of atoms or molecules. When these processes do not involve 

 a change in electronic quantum numbers, they are termed "adiabatic" 

 after Ehrenfest (1916). It has been customary to treat collisions as the 

 impacts of balls, the properties of the balls being refined as the theories 

 gain sophistication. There is, however, considerable merit in a dis- 

 cussion based on the movement of a configuration point on potential- 

 energy surfaces, as outlined in the preceding section. Some examples of 

 the application of this approach have been summarized by Glasstone d al., 

 (1941, pp. 103-107). Eyring et al. (1935) were the first to develop the 

 idea quantitatively, and it has been extended by Gershinowitz (1937) 

 and by Hulburt and his associates (1943, 1950, 1951). Fully (juanti- 

 tative treatments have not been numerous because of the complexity. 

 As a method for understanding, the use of potential-energy surfaces is 

 unexcelled, and we propose to extend in brief formalism the theory of 

 absolute reaction rates to collision processes. No calculational utility is 

 lost in this treatment, since at ultimate simplification the absolute theory 

 reduces to simple collision theory (Eyring, 1935). 



Collisions in which no vibrational degrees of freedom change quantum 

 numbers are governed by Newtonian mechanics, since essentially all 

 translational and rotational energies are available. That is, translational 

 and rotational quanta are so small that they allow very nearly a con- 

 tinuous spectrum of energies. Hydrogen and heavy-metal hydrides are 

 exceptions, because they have a very small moment of inertia and hence 

 large rotational quanta. Vibrational quanta, on the other hand, are 

 usually large, so that only a few discrete energy values E^,{n), in which 

 n is the vibrational quantum number, are available for vibrational 

 degrees of freedom. The correspondence limit provided by classical 

 mechanics does not apply, and such degrees of freedom must be treated 

 by quantum mechanics. The simplest example of an energy exchange 

 involving vibrational quanta is the reaction 



A + XY[E„{n)] -^ A* + XY[E,{n - I)], (1-10) 



in which A is an atom, XF is a diatomic molecule, and A* is a trans- 

 lationally excited atom. If the translational energy before collision is 



