Photochemical Decomposition of Aqueous Permanganate Ion

Abstract

Quantum yields for the photochemical decomposition of aqueous MnO4— have been measured under a variety of conditions using a sensitive method for measuring O2 production, first developed by Pringsheim and co-workers. The quantum yield was found to depend strongly on the wavelength of the light, slightly on temperature at the longer wavelengths, and not at all on the composition of the solution or on light intensity. Additional experiments were carried out which showed that the isotopic composition of photochemically produced O2 was independent of the O-isotopic composition of the solvent water. These results lead to the conclusion that the radiant energy absorbed by the MnO4— ion is converted with high probability into vibrational energy of the ground state, after which the ion dissociates thermally into MnO2 and O2 (in a single act) or cools down without dissociation (the position of the extra electron is uncertain). A possible quantitative theory which predicts fairly successfully the dependence of the quantum yield on wavelength is outlined.