The Function of Water Vapour in the Photosynthesis of Hydrogen Chloride

Abstract

EVIDENCE was presented (B. Lewis and E. K. Rideal, J. Chem. Soc., 129, 583 and 596; 1926) for the view that the photo-expansion of bromine and other halogens in the presence of water vapour (Budde effect) is due to heat liberated by the recombination of halogen atoms set free by the absorption of light quanta. Although absorption of radiation occurs in the dry gas, no Budde effect is observable (J. W. Mellor, J. Chem. Soc., 81, 1280; 1902; Lewis and Rideal, loc. cit.) even when the gas is subjected to an intense source of ultra-violet radiation (E. B. Ludlam, Proc. Roy. Soc. Edinburgh. 44, 197; 1924). This is interpreted to mean that the halogen does not dissociate in the dry state; that the radiation absorbed activates the halogen molecule for a short period of time and is then emitted (presumably as longer wavelengths).