192 RADIATION BIOLOGY 



Thus the reaction proceeds at a rate which depends upon the energy flow 

 from the outside. Usually the quantity of energy transferred to the 

 molecules to be activated surpasses the thermal energy by a high 

 factor, with the result that the energy-rich molecules react in cold 

 surroundings. 



The most important source of energy available to induce the latter type 

 of reaction is light. Reactions caused by irradiation with light are 

 called photochemical reactions. The study of reactions induced by irra- 

 diation with high-energy radiations, such as high-energy photons (X rays, 

 7 rays) or high-speed material corpuscles (electrons, protons, a particles, 

 etc.) is called radiation chemistry. There are great similarities between 

 photochemistry and radiation chemistry, and, on the other hand, great 

 differences. The similarities stem from the fact that both types of radia- 

 tion interact with the electronic systems of the irradiated molecules. 

 The dissimilarities are the result of the exceedingly great difference 

 between the energies of individual quanta of visible and ultraviolet 

 light used in photochemistry and those of quanta of X and 7 rays, 

 or individual kinetic energies of the swiftly moving corpuscles (a and ^ 

 particles, etc.). 



The first chapter of this volume deals \vith the ways in which various 

 high-energy radiations transfer energy to matter when they penetrate it. 

 The next stage of analysis is then treatment of the mechanisms by which 

 the transferred energy effects permanent (or transient) physical, chemical, 

 or biological change. It is the purpose of this article concisely to review 

 the physical principles governing elementary reactions which primarily 

 activated atoms and molecules can undergo. Particular emphasis is 

 accorded the behavior of polyatomic molecules and condensed phases, the 

 phenomena in these systems being the most relevant for application to the 

 problems of biological material. An elementary treatment of atoms and 

 diatomic molecules is included; it may serve as an introduction to the gen- 

 eral subject. Initial energy absorption and kinetics are mentioned only 

 incidentally. (For information on kinetics, reference should be made to 

 the chapter of this or a following volume dealing with the specific type of 

 reaction.) The necessity of dealing with so vast a subject in so limited 

 a space clearly predisposes the treatment to be somewhat cursory. It 

 is hoped that this tendency has been minimized, and that a central 

 objective of the article — to achieve the proper perspective so important 

 to a new and rapidly developing science — has been approached. 



Bibliographic listing is given chiefly to reviews to which the reader may 

 refer for more detailed analysis and bibliography. A number of repre- 

 sentative references to original literature is also included, but no attempt 

 has been made to develop a complete bibliography. In particular, older 

 references are not cited at all. 



By far the best reference for elementary processes in the gaseous phase 



