PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 231 



sisting of the excited molecule itself together with a portion of the solvent 

 adjacent to it. Such internal conversion takes place very soon after 

 excitation in almost all cases. Luminescence occurs (as in the cases 

 mentioned) only for unusual instances in which the "portion" of the 

 molecule or atom that is excited is virtually uncoupled with the liquid 

 environment. If a molecule in solution gains, directly or indirectly, 

 oscillational energy by light absorption which surpasses by much the 

 average thermal energy, it will always lose a considerable part of it to 

 the surrounding molecules of the solvent with which it is permanently in a 

 state of collision. Therefore, the energy utilization of light for photo- 

 chemical processes in condensed systems will never be perfect. A surplus 

 of energy over the amount actually stored as chemical energy must be 

 available to compensate for the losses which must necessarily occur by 

 heat dissipation. Thus, for a simple dissociation process having dissocia- 

 tion energy D, hv must be considerably greater than D to achieve dissocia- 

 tion. Furthermore, even if this condition is fulfilled and two radicals 

 are formed, the quantum yield of this process will be diminished by 

 primary recombination of the radicals because the solvent surrounding 

 the dissociated solute acts as a "cage"; dissociated fragments collide 

 many times under conditions where their recombination energy can be 

 removed by transformation to oscillational energy of adjacent molecules, 

 and will often recombine. There is abundant experimental evidence to 

 support the conclusion that, in many cases, excitation is followed by 

 permanent photochemical change in only a fraction of the events. Of 

 course, this conclusion applies to the result of excitation whether the 

 latter be effected optically or by penetration of high-energy radiation. 

 If the primary radicals do escape from the cage, they will diffuse into the 

 solution and their ensuing reactions with one another and various solutes 

 will . in aggregate produce the final chemical change. The diffusion 

 processes, as well as the elementary chemical reactions between radicals 

 and normal molecules, will obviously be influenced to a greater or lesser 

 extent by the solvent environment. Exceptions to the prediction of 

 small primary photochemical yield resulting from the cage effect occur 

 where one of the radicals can react immediately with the solvent, in 

 which case the yield approaches unity, or where the radicals are formed 

 with especially high kinetic energy sufficiently great to enable them to 

 escape from the cage. 



With substances that are fluorescent in solutions, such as many dye- 

 stuffs, internal conversion of the excitation energy must be very slow. 

 The excitation energy is, therefore, photochemically available for a period 

 of the order of magnitude of 10"^ second. Experiments on the "phys- 

 ical" quenching of this fluorescence supply information on the types and 

 numbers of collisions which transform the excitation energy into heat, 

 and experiments on sensitized photochemical reactions provide informa- 



