Diffusion Effects in the Photochemistry of Solid Films

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Little work has been done, either experimentally or theoretically, on the problem of photochemical reaction kinetics in the solid phase, although this field is of practical importance in connection with the weathering of polymeric films and sheets. Reaction kinetics in solid films are greatly influenced by diffusion phenomena. The Laplace transformation technique offers a convenient method of solving many problems concerning the interaction of diffusion behavior with reaction kinetics. The method is illustrated for the diffusion of a gaseous product from an irradiated film, with and without reaction rate dependence on product concentration, and for the diffusion of a gaseous reactant into an irradiated film, with and without reaction rate dependence on reactant concentration. In each case the expression for dissolved gas concentration across the thickness of the film is computed, and the quantum yield for the reaction involved is expressed as a function of film thickness, diffusion coefficient, rate of light absorption, and initial conditions. Equations for the effect of temperature variation and of intermittent light are derived in a few instances.