Photolysis of hydrogen iodide at cryogenic temperatures

Abstract

The photolysis, using 253.7 nm radiation, of HI in thin films has been investigated at 77 and 85 K. Initially the quantum yield ϕ_HI= 2.0. However, as reaction proceeds the rate falls off rapidly and, with films of sufficient thickness, reaction does not go to completion but approaches a photostationary state. This self-inhibition is attributed to the light-filtering action of some product species. A mechanism involving photolysis of polymeric aggregates is proposed. During photolysis part of the hydrogen which has been formed diffuses out of the solid into the gas phase. The rate of evolution, unlike the overall rate of hydrogen formation, remains almost constant throughout photolysis but, below 90 K, evolution stops immediately 253.7 nm irradiation is stopped. Thus, in this temperature region, diffusion is not a simple isothermal process but occurs as a result of local heating of the film by absorbed u.-v. radiation. The diffusing species is molecular hydrogen.