PHYSICAL PRINCIPLES OF CHEMICAL REACTIONS 229 



molecule before being removed in subsequent impacts. (The same argu- 

 ment applies, of course, to normal association reactions, for which neither 

 reactant is excited.) Other possible processes, of which a few cases have 

 been observed, are induced predissociation and induced internal conver- 

 sion. An example of the latter is the photochemical isomerization of 

 1-butene to 2-butene, which occurs following electronic excitation of 

 1-butene only during a collision of the excited molecule wdth a normal one. 

 In general, if an excited molecule is brought by internal conversion to a 

 metastable state, the remaining excitation energy will be available for a 

 longer time, and the chance for a collision of the second kind becomes 

 proportionately greater. 



It is usually very difficult to analyze reactions involving excited poly- 

 atomic molecules, because of the complexities of the associated spectra 

 and the intricacies of the various processes involved. However, much is 

 known, relatively, of the behavior of certain special classes of unsaturated 

 organic substances with conjugated double bonds (Sect. 3-4). 



3-4. ELEMENTARY PROCESSES IN CONDENSED SYSTEMS 



3-4a. Liquids and Liquid Solutions: Weak Interaction with the Medium. 

 The characteristics of excitation, ionization (see Sect. 4-4), and ele- 

 mentary chemical processes in liquid solutions, and particularly in 

 aqueous solutions, comprise one of the most important topics upon which 

 a fundamental understanding of radiobiological phenomena must ulti- 

 mately be based. 



In this and the following subsection, solutions are classified according 

 to whether the solute exhibits optical properties which can be inter- 

 preted as modifications of the corresponding properties in the gaseous 

 state, or whether these properties are more properly considered to be 

 characteristic of the solute-solvent system and without gaseous analogue. 



Examples of "weak interaction" in this sense are rare earth ions, 

 mercury atoms, or dyestuffs dissolved in water; halogen molecules dis- 

 solved in organic solvents; and aromatic hydrocarbons dissolved in one 

 another. In the brief space allotted here it will be possible only to outfine 

 broadly the optical and photochemical phenomena of fiquids and liquid 

 solutions. 



Absorption spectra of dissolved substances are comparatively easy to 

 measure, and there exists a great wealth of recorded data on absorption 

 spectra of solutions (cf., for example, Landolt-Bornstein, 1951). Inter- 

 pretation of these data is another ciuestion, and relatively few of them 

 are understood in detail, in the same sense that spectra of gases are 

 understood. 



Most absorption spectra of liciuids are continuous, and almost all of the 

 remainder exhibit only a diffuse band structure, the rotational structure 

 being missing. This is a consequence of "Stark broadening" of the 



