232 RADIATION BIOLOGY 



tion on the collisions which transform it into chemical energy (Pringsheim, 

 19-49). The excitation energy can also be transferred from one molecule 

 to another by collisions of the second kind between two molecules, or even 

 over such great distances that the term coUision does not properly apply 

 (Sect. 3-4d). 



3-4b. Liquids and Liquid Solutions: Strong Interaction with the Medium. 

 In cases such as ions in aqueous solution, electronic excitation can be 

 understood only by considering the absorbing system to be the ion 

 together with its immediate solvent environment. Thus, even for 

 "monatomic" ions, for example the colored ions such as those of Fe, Cu, 

 Cr, Mn, Co, Ni, and ions which absorb in the ultraviolet such as the 

 halide ions, the absorbing system is a "polyatomic molecule" and the 

 spectra are continuous. Solutes that possess "weak interaction" spectra 

 must always have "strong interaction" portions of the spectrum further 

 to the ultraviolet; thus, the color of the permanganate ion arises from an 

 internal transition which is not much influenced by the environment (the 

 visible spectrum is approximately the same in crystal and dissolved 

 phases), but further to the ultraviolet the permanganate ion exhibits 

 absorption for which the neighboring solvent layer plays an essential role. 



Many electronic transitions of the "strong interaction" type can be 

 understood in an approximate manner as electron transfer processes in 

 which an electron is removed from the solute ion or molecule and trans- 

 ferred to a final state in which it is bound to a different unit in the 

 environment; for example, in the case of the halide ions and probably 

 most other negative ions and many positive ions in aqueous solutions, it is 

 bound to the solvent layers about the absorbing entity (Rabinowitch, 

 1942; Platzman and Franck, 1952). It is usually the case then that 

 fluorescence does not occur; internal conversion of the excitation energy 

 to atomic vibrations is very cpick; and photochemical utilization of the 

 energv, if it is possible at all, must result from reaction of the excited 

 system with the solvent (for example, by dissociation of the solute- 

 solvent "polyatomic molecule") or from colUsion with a solute molecule 

 within a very short time after excitation. 



3-4c. Solids. Solids may be classified according to structural character 

 (single crystals, polycrystalUne solids, glasses, etc.) or according to elec- 

 tronic character (metals, semiconductors, insulators, etc.). Modern 

 physics has made possible, to a great extent, a detailed understanding of 

 their structural, optical and electronic properties, but space does not 

 permit discussion of these matters here. On the other hand, compara- 

 tively little investigation has been made of photochemical or radiation- 

 chemical reactions in solids. Their inclusion in the content of this 

 article is not, however, a mere formahty. It is probable that many of the 

 essential organizational properties of biological systems derive from the 

 same principles as do the striking organizational properties of crystals and 



