98 PRIMARY PROCESSES 



photochemical reactions, commonly those of photons of visible or ultra- 

 violet light and therefore in the region of 2-10 ev, lie just in the range 

 of the most likely transitions of molecular electronic systems. Whereas 

 in photoeffects the quanta are absorbed in single events, in high-energy 

 radiation effects the energy loss occurs gradually, in hundreds or many 

 thousands of steps. Hence use of monochromatic light for a photochemi- 

 cal reaction ensures, in general, the excitation of a unique excited state, 

 but monoenergetic high-energy radiation — for example, homogeneous al- 

 pha or beta rays — always produces a very great number of different 

 types of excited and ionized molecules of quite widely varying energy. 



Another contribution to the complexity displayed by the primary 

 processes of radiation chemistry and biology arises from the fact that 

 many, and usually the majority, of the products are formed, not directly 

 by the incident radiation, but indirectly by secondary, tertiary, etc., 

 radiations which the primary radiation produces. It would evidently 

 be extremely difficult to gain full knowledge of the products of absorp- 

 tion of even the simplest high-energy radiation. 



The nature of the primary processes has already been discussed by 

 the first panel of this symposium. These primary processes are almost 

 all separate excitation or ionization events in isolated atoms or molecules 

 of the medium. The processes can be described in a general way, and 

 such a description is invaluable background for understanding chemical 

 and biological effects of radiation. We may be permitted to emphasize, 

 however, that quantitative prediction of the physical effects is within 

 the realm of possibility, at the present time, only for gaseous media com- 

 posed of monatomic molecules, because a necessary basis for comprehen- 

 sive treatment of the physics of a radiation process is the knowledge 

 (that is, of constitution, energy, stability, and other specifications) of 

 the possible stationary states of the system. Such information on sta- 

 tionary states, obtained principally from spectroscopic investigation, is 

 available in detail only for atoms: excited and ionized states of diatomic 

 molecules are far less extensively known, owing to the overwhelmingly 

 greater difficulty in interpreting the spectra; in the case of polyatomic 

 molecules our knowledge is hardly more than in its infancy, and great 

 progress in the near future is not to be anticipated. Some knowledge 

 of ionized states, but not of excited states, is obtainable from mass 

 spectrographic studies for both atoms and molecules; however, it is of 

 highly restricted content. It would be a sophistry to deny that contem- 

 porary radiation physics, although it provides most of the information 

 ordinarily required by the -physicist, customarily disregards consequences 

 of chemical binding, and is scarcely more than a general guide to the 

 understanding of the details of the primary processes in chemical and 



