CHAPTER 1 



Energy Exchange in Photoreactionsi 



RUFUS LUMRY^ AND HeNRY EyRING 



The University of Utah, Salt Lake City 



Introduction: Scope of this chapter — Potential energy and light absorption — Black- 

 body radiation — Potential-energy diagrams. Internal conversion of energy: Conditions 

 for intersection of potential-energy surfaces — Types of internal-conversion processes — 

 Conjugated molecules. Energy transfer during adiabatic collisions: Absolute theory of 

 collisions — Energy transfer involving rotational degrees of freedom — Adiabatic processes 

 of vibrationql-energy exchange — Collisions of polyatomic molecules. Energy exchange 

 in diahatic processes: Types of diabatic processes — The quenching of fluorescence — 

 Energy transfer between electronic degrees of freedom — Quenching with vib rational-energy 

 transfer. Chemiluminescence. Energy transfer in biological reactions. References. 



1. INTRODUCTION 



1-1. SCOPE OF THIS CHAPTER 



The photoreactions of everyday importance to life are associated with 

 wave lengths of electromagnetic radiation lying in the region from about 

 2000 to 10,000 A. At short wave lengths, absorption of radiation by 

 most molecular species will result in permanent, nonuseful decomposition. 

 The shorter wave lengths are largely removed from the sun's radiation by 

 atmospheric absorption. At long wave lengths, energy will be taken up 

 by single vibrational, rotational, or translational degrees of freedom and 

 will be rapidly dissipated, not to produce chemical reaction, but in the 

 trivial process of adding to the thermal energy of the system. Common 

 processes in the ultraviolet are not likely to be apparent, because radi- 

 ation in this region is not visible. A few important biological processes 

 depend on radiation in this region. For instance, the formation of vita- 

 min D from its provitamin is a well-publicized process. The rate of 

 mutation production may also depend on the damaging effect of ultra- 

 violet radiation on genetic structure. In general, the more colorful proc- 

 esses caused by visible radiation have attracted more biological attention. 

 Vision itself, photosynthesis, growth mechanisms in plants, and numer- 



' Contribution from the Laboratory for the Study of Photosynthetic Processes, 

 supported by the Atomic Energy Commission. 



^ Now at School of Chemistry, University of Minnesota. 



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