PULSED-LASER-INDUCED CHARGE-TRANSFER REACTIONS IN AQUEOUS TIO2 COLLOIDS - A STUDY OF THE DEPENDENCE OF TRANSIENT FORMATION ON PHOTON FLUENCE

The formation of charge-transfer transients in the reduction of methyl viologen and the oxidation of halides in aqueous unprotected TiO2 colloids has been studied as a function of the photon fluence (pulse energy) of 10-ns 355-nm laser pulses. Nonlinearity of the observed dependence is interpreted by a model based on depletion of surface-adsorbed reactants at high photon fluences. It is shown that this model is able to give a consistent explanation of data obtained at various concentrations of reactant and semiconductor. Results of model calculations include adsorption properties of reactants and information concerning the factors that influence quantum yields of transient formation. The alternative model introduced by Nosaka and Fox is also applied to the data; it is shown that depletion of adsorbed reactants is a consequence of this model also as long as the assumption of reactant adsorption is retained.