36 KADIATIOX BIOLOGY 



of elliyk'iu' (^'I'uylor and Enioliius, li)31j induced by tlie prcdisfjuciatiou 

 of ammonia: 



NHs + hv^ XH2 + ft. 



When a mixture of hydrogen, oxygen, and ammonia, at a moderate!}' 

 elevated temperature, is illuminated with light of wave length 2200 A or 

 shorter, ammonia is decomposed and water is formed. The cjuan^um 

 yield for the formation of water increases from about 25 at 290°C to 

 approximateh^ 380 at 405°C. At 420°C, irradiation of the system results 

 in an explosion. The kinetics are complex (Lewis and von Elbe, 1938) 

 and probably involve the amide radical as well as the hydrogen atom. 



If a mixture of carbon monoxide and chlorine is illuminated with light 

 which is absorbed by the chlorine, a chain reaction ensues, the product 

 of which is phosgene. The kinetics of the reaction are complex, but the 

 primary act is certainly the dissociation of chlorine and the radicals CI 

 and COCl are involved in the secondary reactions. If an excess of oxy- 

 gen is added to the system, the formation of phosgene is suppressed, and 

 the predominant process becomes the sensitized formation of carbon 

 dioxide (Rollefson and Burton, 1939, pp. 313-319). The quantum yield 

 of carbon dioxide formation is large and is a complex function of tem- 

 perature and the partial pressures of the reactants. A numl)cr of reac- 

 tions of this general type have been studied, but the mechanism of none 

 of them is completely understood. 



The photolysis of ethyl iodide is sensitized (West and Miller, 1940; 

 West, 1941) by naphthalene and a number of its derivatives. The direct 

 photolysis of ethyl iodide occurs both in the gas phase and in solution. 

 In hexane solutions the quantum yields corresponding to wave lengths 

 3130 and 2537 A are about 0.30 and 0.40, respectively. The quantum 

 yield of the naphthalene-sensitized process is about 0.30 for cither wave 

 length. The maximum fluorescence efficiency of naphthalene in hexane 

 solutions is approximately 0.15. As was clearly stated by West (1941), 

 this demonstrates that ethyl iodide can interact with a nonfluorescent 

 excited state as well as with the fluorescent excited state of the naphtha- 

 lene molecule. The yield of the sensitized reaction is independent of 

 the naphthalene concentration but falls off to small values when the e{\\y\ 

 iodide concentration is decreased much below 10~'- M. These results, 

 as well as observations on the effect of changing the \iscosity of the 

 solvent, show that the sensitization is a coUisional process, that the 

 efficiency of such collisions in producing the reaction is high (probably 

 greater than 0.1), and that the collisions in the condensed system occur 

 in bursts (p. 15). All these data are consistent with the following 

 mechanism, which is strikingly similar to the mechanism here offered 

 as an explanation (p. 28) for the photoautooxidation of aromatic hydro- 

 carbons (Howen and Williams, 1939): 



