PHOTOCURRENT KINETICS AT THE ELECTRON-EMISSION FROM METAL INTO ELECTROLYTE SOLUTION .10. DISCHARGE OF SHORT-LIVED INTERMEDIATE SPECIES

Electrode reactions of intermediate species (IS), generated by a short pulse of laser photoemission (LPE), result in the time-dependent chan ge of emitted charge Q(t). Analytical expressions for the kinetic curv es Q(t) are derived by solving non-stationary diffusion equations for e(aq)(-) and IS. For the IS adsorption Gibbs energy less than -25 kJ m ol(-1), kinetic curves art exponential over the very wide range of ele ctrode reaction rate constant W, from 1 up to 10(7) s(-1). The depende nce Q(t) alpha t(-1/2) is typical for the case of activated adsorption of IS or their discharge from the non-adsorbed state. Voltammograms o f IS generated by pulse radiolysis, modulated photolysis and pulsed or alternating photoemission current are demonstrated to be described by similar expressions. The difference between half-wave and equilibrium potentials depends on the reactant and product lifetimes and rates of desorption. A characteristic trapezoid of Tafel lines is introduced a s a new way to characterize completely the kinetics of two-electron el ectrode reactions. The relations obtained were applied to the analysis of hydrogen evolution reactions and carbon dioxide and formaldehyde r eduction, where hydrogen atoms and organic radicals HCO2 and CH2OH ads orbed on a mercury electrode participate as IS.