The Effect of Temperature on the Predissociation of Photoactivated Acetaldehyde Molecules

Abstract

The relative efficiencies of production of free radicals in acetaldehyde by the absorption of light of the wave‐lengths 3132 and 2652A have been studied by observing the quantum yields of the photochemical reaction at high temperatures. It has been found that at room temperature more radicals are produced by the absorption of λ = 2652A than by λ = 3132A. As the temperature is raised, the yield of radicals at the longer wave‐length increases relative to that at 2652 until at 100°C the two are equal. At higher temperatures the longer wave‐length has the greater efficiency for the production of radicals. This change with temperature is accounted for by assuming that at λ = 3132A the dissociation is aided by thermally excited vibrational energy. In accordance with this assumption it is found that as the temperature is raised the fluorescence, which is easily visible at room temperature, fades in a manner which is roughly parallel to the increase in dissociation. There is no evidence for such a thermally induced predissociation with λ = 2652A. The lower efficiency of production of free radicals at λ = 2652A is believed to be due to a difference in the electronic states reached by acetaldehyde excited by the two wave‐lengths.