PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 195 



substances but complicated by the intricate molecular structure. Since 

 even the static structural properties of the simplest biological systems are 

 not fully understood, and properties of excited and ionized states are to a 

 great extent unknown, it is obvious that primarj^ processes in radiobiology 

 can be analyzed in only the most general terms. This situation also 

 reflects, and is in a way an extension of, the greatly heightened complexity 

 encountered in photochemistry or in radiation chemistry when dealing 

 with a liquid or solid phase, as compared to a gaseous phase. 



Another important aspect of biological media is their composite nature. 

 Usually there are present, in addition to the principal biological compo- 

 nent, other components such as substrate and water, and the radiation 

 affects them all indiscriminately: in the case of high-energy radiation, 

 for example, the energy absorbed per unit volume is simply approximately 

 proportional to the density. Since the primary effects of the radiation on 

 these other components may often be transmitted directly or indirectly so 

 as to affect the component of primary interest, radiobiological reactions 

 exhibit an added and serious interpretative difficulty. 



2. THE PRIMARY PROCESSES 

 2-1. THE PRIMARY PROCESSES OF PHOTOCHEMISTRY 



Primary processes in photochemistry being invariably electronic excita- 

 tion or ionization of atoms or molecules, their discussion can be deferred 

 to Sects. 3 and 4 following. It should be noted here that practical limita- 

 tions have restricted investigations to accessible spectral regions, and to 

 absorption processes having appreciably strong transition probability. 

 Thus the interesting region of the vacuum ultraviolet (wave length below, 

 say, 1800 A) is largely unexplored photochemically because of great 

 difficulties with light sources, detectors, and windows. Photochemical 

 consequences of light absorption in weak transitions have not been 

 studied (in gases, at least) because, with light sources of normal intensity, 

 not enough effect can be achieved for accurate measurement. (A special 

 case, namely reactions of metastable excited atoms, has often been 

 studied by producing the metastable atoms indirectly.) 



It is not necessary that the reactant itself absorb the light. An impor- 

 tant class of photochemical reactions, called photosensitized reactions, 

 exploits the light absorption by a small admixture of chemically inert 

 substance — in the gaseous phase usually a metal vapor, in the liquid usu- 

 ally a dye — which absorbs in a region in which the reactant itself is 

 transparent and affects the reactants in subsequent stages. Such 

 phenomena will be described. 



The over-all photochemical reaction is a dynamic synthesis of two 

 classes of elementary processes : reactions between excited or ionized atoms 

 or molecules, and normal ones (which may proceed according to rate laws 



