PHOTOCHEMISTRY 35 



solid tiHigstic oxide. If pure hydrogen is substituted for the mixture of 

 gases, there is no reaction. The reaction steps are as follows: 



Mel-cury vapor is a l)etter-kno\vn sensitizer for the dissociation of hydro- 

 gen (Noyes and Leighton, 1941). The first resonance radiation of mer- 

 cury, wave length 2537 A, corresponds to an energy of 112 kcal/einstein, 

 which is only slightly more than is necessary to dissociate molecular 

 hydrogen. It should be expected that the reaction 



H., + Hg*(()^Pi) -^ Hg((3'Po) + 2H 



should be very efficient. Although the interaction between an excited 

 mercury atom and a hydrogen molecule is indeed very probable, HgH 

 appears to be one of the products: 



H2 + Hg* ^ HgH + H. 



If other reactant gases, such as carbon monoxide or ethylene, are present, 

 the hydrogen atoms initiate a series of reaction steps leading to a variety 

 of products. Many such mercury-sensitized reactions have been 

 studied. 



Photochemical cis-trans isomerizations are sensitized by iodine (Ber- 

 thoud and Urech, 1930; Dickinson et al, 1949). The sensitized reaction 

 is a short chain process with an appreciable heat of activation. The pri- 

 mary act is the photodissociation of molecular iodine. Iodine atoms can 

 add, with an appreciable heat of activation, to carbon atoms adjacent to 

 the double bond. This opens the double bond, permitting rotation of the 

 groups. Subsecjuently the iodine atom can split off. This mechanism is 

 summarized in the following equations, in which C and T stand, respec- 

 tively, for the CIS and trans form of the molecule: 



I2 + hv-^2\, 



i+ T^fi, 



i + c ^ CI, 



CI ^ fi, 



2\ -^ I2. 



In some systems an absorbing compound, undergoing a permanent 

 photochemical reaction with a yield of about 1, simultaneously induces a 

 chain reaction between other reactants. The chain reaction so over- 

 shadows the inducing reaction, that the whole process may be thought of, 

 loosely, as a sensitized photochemical reaction. Examples of this type 

 are the oxidation of hydrogen (Farkas et al, 1930) and the polymerization 



