PHOTOCHEMISTRY 



27 



of these studies are far from simple. It appears to be fairly definite that 

 two different kinds of primary acts can occur. The excited molecule can 

 dissociate either into radicals or into two such stable molecules as a hydro- 

 carbon and carbon monoxide. The formation of the radicals is probably 

 a process of predissociation, and the production of molecules, the result 

 of internal conversion. Croton aldehyde, at temperatures below 150°C, 

 is photochemically stable in spite of the facts that it is nonfluorescent and 

 that its absorption is continuous. Although this failure to react is con- 

 ceivably the result of rapid recombination of radicals formed in the pri- 

 mary act, it seems much more likely that the energy of excitation is lost 

 by way of an act of internal conversion, followed by degradative colli- 

 sions, i.e., collisions of the second kind, with surrounding molecules. 



The photochemical decomposition of formaldehyde is probably as sim- 

 ple a reaction of this type as has been studied. Although the results of 

 the several investigations of this reaction (Steacie, 1946) do not agree in 

 all particulars, the broad outline of the mechanism appears to have been 

 reasonably well established. The products of the reaction are carbon 

 monoxide and molecular hydrogen. At 110°C the ciuantum yield is 

 approximately unity for wave lengths from 2600 to 3500 A. At the 

 longer wave lengths the absorption spectrum shows fine structure but 

 corresponds to a region of predissociation in the shorter wave-length 

 range. The yield increases with increasing temperature, reaching a 

 value of about 100 at 350°C. Hydrogen atoms can be detected under all 

 experimental conditions, but there is some evidence that there is an appre- 

 ciable direct formation of molecular hydrogen when the gas is illuminated 

 with light in the longer wave-length region. The following mechanism 

 seems to be compatible with the published results: 



(1) HCHO + hv— CHO + H (chief primary step), 



(2) HCHO + hv -^ [HCHO] -^ CO + H, (primary step at long 



wave lengths), 



(3) HCO ^ H + CO (chain-carrying secondary 



step), 



(4) H + HCHO ^ H2 + CHO (chain-carrying secondary 



step), 



(5) M + 2H -^ H2 + AI (chain-breaking secondary 



step) . 



Steps (3) and (5) can occur either in the gas phase or by diffusion to the 

 wall. For the gas-phase reaction the heat of activation of step (3) is 

 about 13 kcal. There are, of course, a number of other possible steps, but 

 these five are sufficient to explain the available data. 



REACTIONS OF MOLECULAR OXYGEN 



In biological systems those photooxidatix'e reactions which involve 

 molecular oxygen are by far the commonest. The primary step of such a 



