On the photolysis of simple anions and neutral molecules as sources of O⁻/OH, SO_x⁻ and Cl in aqueous solution

This contribution examines the aqueous phase photolysis processes of simple anions such as nitrate, nitrite, peroxodisulfate and neutral molecules such as H₂O₂. The review includes new results on absolute effective quantum yields for the photodissociation processes of NO₃⁻, NO₂⁻, S₂O₈²⁻, HSO₅⁻, S₂O₆²⁻, HOCl, and chloroacetone in an aqueous solution. The quantum yields for the photolysis of nitrate and nitrite have also been determined as a function of temperature. Models to interpret the wavelength and the temperature dependencies of the quantum yields for the different systems are discussed and a simple model treatment is developed to quantify the effects of (i) impulse conservation, (ii) electrostatic interaction (e.g., ion–dipole, dipole–dipole and coulomb interaction between the photofragments directly after photolytic fragmentation), and (iii) diffusion and recombination. The combined impulse-interaction-diffusion (IID) model is compared to the experimentally observed effective radical formation quantum yields and reasonable agreement is found for a number of systems. It is shown that the temperature dependencies for effective quantum yields of photolysis processes in aqueous solution are not only governed by the temperature dependence of the viscosity of water but also determined by the temperature dependence of the rate constants of the photofragment recombination reactions.