Photocurrent kinetics at the electron emission from metal into electrolyte solution X. 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 change of emitted charge Q(t). Analytical expressions for the kinetic curves 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 mol −1 , kinetic curves are exponential over the very wide range of electrode reaction rate constant W, from 1 up to 10 7 s −1 . The dependence Q(t) ∝ t −12 is typical for the case of activated adsorption of IS or their discharge from the non-adsorbed state. Voltammograms of 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 as a new way to characterize completely the kinetics of two-electron electrode reactions. The relations obtained were applied to the analysis of hydrogen evolution reactions and carbon dioxide and formaldehyde reduction, where hydrogen atoms and organic radicals HCO 2 and CH 2 OH adsorbed on a mercury electrode participate as IS.