ENERGY EXCHANGE IN PHOTOREACTIONS 35 



considerable difference in activation energies. These and similar proc- 

 esses substantiate the previously discussed belief that vibrational energy 

 migrates with ease throughout molecules. There is no apparent require- 

 ment based on geometry except perhaps in large molecules, in which low- 

 energy-reaction regions are isolated from chromophoric groups by inade- 

 quate coupling through weak or too few bonds. 



There are a few general rules for efficient energy transfer in adiabatic 

 collision processes, owing, as we have seen, to the properties of the 

 potential-energy surfaces involved: 



1. Chemical affinity favors energy transfer. An upper limit of affinity 

 is clearly compound formation between the participants, in which case 

 the migration of vibrational quanta goes on under the most efficient 

 conditions. 



2. High relative velocity of the atoms primarily concerned in a given 

 collision process is generally favorable to collisional transfers, since acti- 

 vation (collision) energies are required. In contrast to the situation in 

 chemical reactions, energies in excess of precise values may reduce rather 

 than increase the rate of migration. 



3. Energy transfer is most probable when the fewest degrees of free- 

 dom change quantum numbers. 



4. ENERGY EXCHANGE IN DIABATIC PROCESSES 



4-1. TYPES OF DIABATIC PROCESSES 



By analogy with the term "adiabatic," "diabatic" processes are 

 defined as those which occur with a change in electronic quantum num- 

 ber. For convenience in treating photoinduced processes, we divide 

 these into two classes. In the first class are included those reactions of 

 molecules which occur unimolecularly after the absorption of radiation. 

 Excitation is to an unstable electronic state or to a stable state from 

 which internal conversion takes place rapidly and spontaneously. In 

 the latter instances the vibrational energy thus produced is dissipated in 

 unimolecular reaction or as heat during the collisions that follow. For 

 convenience we also include those cases in which internal conversion is 

 induced by perturbations of other molecules that do not profit by energy 

 gain or reaction from the process. For instance, the effectiveness of ions 

 as quenchers is frequently attributed to their perturbing reaction (see 

 Sect. 2-1). The migrations of vibrational energy in the excited state 

 preceding internal conversion, as well as the fate of vibrational energy 

 following the crossing of potential surfaces, can be treated at best only 

 very approximately by the methods of Sect. 2. The problem is further 

 complicated on upper surfaces by the lack of knowledge of the position 

 of crossing points. 



