CHAPTER 3 



Physical Principles Underlying Photochemical, 

 Radiation-Chemical, and Radiobiological Reactions 



James Franck 



Institute jor Radiobiology and Biophysics {Fels Fund),* 

 University of Chicago 



AND Robert Platzman 



Department of Physics, Purdue University 



Introduction: Scope of this article — Photochemical reactions — Radiation-chemical 

 reactions — Radiobiological reactions. The primary processes: The primary processes of 

 photochemistry — The primary processes of radiation chemistry — The primary processes of 

 radiobiology. Reactions following excitation: Elementary processes involving excited 

 atoms — Elementary processes involving excited diatomic molecules — Elementary processes 

 involving excited polyatomic molecules — Elementary processes in condensed systems. 

 Reactions following ionization: The ionization process, and the role of ionization in photo- 

 chemical reactions — Survey of ionization phenomena produced by high-energy radiations — 

 Elementary processes following ionization in gaseous systems — Elementary processes 

 following ionization in condensed systeyns — Multiple ionization — The significance of 

 density of ionization. General reinarks on applications to biology: Indirect chemical 

 effects of high-energy radiations on biological material — Direct chemical changes produced 

 by high-energy radiations in inolecules of biological importance. References. 



1. INTRODUCTION 



V 1-1. SCOPE OF THIS ARTICLE 



Chemical reactions may be classified into two types. The first type 

 comprises common or "thermal" chemical reactions. These occur spon- 

 taneously at temperatures at which the energy of thermal motion of the 

 reactants is great enough to provide the energy required for the particular 

 reaction to proceed during the time of observation. The second type of 

 reaction takes place in chemical systems in which the molecular thermal 

 energy falls far below the minimum energy needed to initiate the reaction. 

 In this case, however, energy coming from an external source may transfer 

 sufficient energy to some of the molecules to render them able to react. 



* These studies were aided in part by a contract between the Office of Naval 

 Research, Department of the Navy and the University of Chicago (NR 119-272). 



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