CHAPTER 1 



Photochemistry 



Robert Livingston 

 School of Chemistry, University of Minnesota, Minneapolis 



Introduction. Primary steps: Absorption — Franck-Condon principle — Direct optical 

 dissociation — Half life of the excited state — Predissociation — Internal conversion — Phos- 

 phorescence and long-lived fluorescence — Long-lived energetic states — Quenching of excited 

 states — Transfer of excitation — Solvent effects — Cage effect — Photochemical transfer of 

 electrons or protons to the solvent. Secondary steps: Bimolecular steps — Unimolecular 

 steps — Tennolecular steps — Diffusion-controlled processes. Mechanism of complex 

 reactions: General problem — Steady-state approximation. Examples of the principal 

 types of photochemical reactions: Decomposition reactions — Reactions of molecular 

 oxygen — Polymerization and dimerization — Intramolecular changes — Sensitized reac- 

 tions. References. 



INTRODUCTION 



The development of photochemistry as a quantitative science was made 

 possible by the discovery of the Einstein photochemical equivalence law 

 and by the accumulation of spectroscopic knowledge. The equivalence 

 law can be stated as follows: a photon can induce a photochemical reac- 

 tion only by being absorbed and, on being absorbed, will activate one and 

 only one molecule. The spectroscopic behavior of practically all (stable) 

 diatomic molecules is well understood. In studying the optical proper- 

 ties of complex molecules, it is usually necessary to be guided by general 

 principles and qualitative analogies. In principle, there is little difference 

 between photochemical reactions utilizing visible light and those produced 

 by ultraviolet radiation. 



The observed change in a photochemical reaction, as in a thermal reac- 

 tion, is the result of the concurrence of a number of simple reaction steps. 

 A set of reaction steps, which is consistent with all available information 

 about a reaction, is called the "mechanism of the reaction." It is con- 

 venient to divide the steps which constitute the mechanism into two 

 groups, called "primary" and "secondary" steps. The primary steps 

 are those chemical or physical processes which are the direct consequence 

 of the absorption of the photon and which involve only the absorbing 

 species (and possibly the solvent). Reactive molecules, radicals, or 

 atoms are produced by the primary steps. These reactive entities can 



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