Electron transfer reactions sensitised by colloidal cadmium sulphide: Effect of colloid concentration and flash intensity on rate

Abstract

Using flash photolysis, the kinetics of recovery of the photoinduced transient bleaching in the absorption spectrum of colloidal CdS in the presence of a quencher (O 2 or methylviologen) are studied as a function of colloid concentration and flash intensity. As predicted by a “first-order” kinetic model, which assumes that the quenching reaction is activation-controlled, using O 2 as the quencher and ascorbic acid as the electron donor, when the colloid concentration is varied, the normalised transient decay traces superimpose. The shape of the decay curves is as predicted by the “first-order” kinetic model for a log-normal distribution of particle size of width (ρ) = 1. For the CdS/ascorbic acid/O 2 photosystem, the shape of the transient decay curve is independent of the flash intensity, a behaviour predicted by the “first-order” model. In contrast to the above results, using methylviologen as the quencher and cysteine as the sacrificial electron donor, the decay traces broaden as the flash intensity is increased. This behaviour is as predicted by a “zero-order” kinetic model, which assumes the reaction is diffusion-controlled, as well as a log-normal distribution in particle size.