A note on the quantum yield of the photosensitized decomposition of water and of ammonia

Abstract

When hydrogen atoms are produced by collisions between hydrogen molecules and excited mercury atoms in mixtures containing gases other than hydrogen, there is always a complication introduced by the fact that these gases may also react with the excited Hg atoms. This difficulty appeared when studying the kinetics of the exchange reactions of deuterium atoms with hydrides. In the latter reactions exchange may occur by the dissociation of the hydride and subsequent recombination of the fragments with atomic deuterium. With the object of finding the maximum possible rate of this type of exchange, measurements have been made of the quantum yields of the decomposition of H 2 O, D 2 O, and NH 3 at different temperatures. These measurements are of importance, for in the water exchange experiments it was not possible at higher temperatures to decide unequivocally that the exchange went by way of the chain reaction D + H 2 O = HDO + H, H + D 2 = HD + D. The experimental arrangements were similar to those described for the exchange experiments with the exception that a Pirani gauge was employed to measure the pressure of gas after the water or ammonia had been frozen out by means of liquid air. Droplets of mercury were distilled on to the surface of the tube leading to the reaction vessel to ensure that mercury vapour was present throughout the run.