Spectrum and Dissociation Processes in Water Vapor

Reactions (44) and (45) are energetically possible over the entire absorption range for water vapor while reaction (46) is energetically possible at 1345 A. Barrett and Baxendale (3) state that reaction (44) is not possible energetically above 1800 A. However, this conclusion was based on a discussion by Herzberg (40) applying to formation of O (ID) for 

The careful studies of Chen and Taylor (22) on the photolysis at about 1650 A. have established the dependence of products on the experimental conditions. In static systems, hydrogen peroxide was not found, and the decomposition products were hydrogen and oxygen in a mole ratio of 2 1. The quantum yield under these conditions was about 0.01. In a flow system, hydrogen peroxide could be collected in a liquid nitrogen trap past the illumination zone, and evidence was obtained that it was not formed in the gas phase but in the trap itself. Under these conditions quantum yields as high as 0.3 were obtained, based on the sum of H2 and H2O2 formed. 

On the assumption that the primary reaction was (45), the following surface or three body reactions were considered to occur  

This reaction is 19.2 kcal./mole exothermic. However, as pointed out by Friel and Krieger (31), according to the Hirschfelder rules (43) this reaction should have an activation energy of about 28% of the sum of the energies of the two bonds being broken or about 57 kcal./mole. Friel and Krieger point out that the reaction 

There would appear to be no difference in the reactions expected photo-chemically as the primary products are H atoms and OH radicals in both the electric discharge and on irradiation. Chen and Taylor (22) state that there is no evidence for oxygen atoms either in the photolysis or in the decomposition of water vapor in an electric discharge. However, the secondary formation of O atoms (2) and the formation of ozone (31, 50) in an electric discharge through water vapor have been demonstrated. It might be expected that under the proper experimental conditions similar results could be obtained photochemically. 

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