Laser-induced temperature-jump coulostatics for the investigation of heterogeneous rate processes: Theory and application

Abstract

A coulostatic perturbation of an electrode surface effected by a laser-induced temperature jump is described. A thin foil or film electrode exposed to dielectric (air, or glass backing) on one side and solution on the other is irradiated by a laser pulse from the dielectric side. Non-reflected photons are absorbed in a thin layer of the metal at the metal/dielectric interface and thermalize virtually instantaneously. The high thermal diffusivity in the metal causes rapid heating of the metal/solution interface and a concomitant change in the open-circuit potential of the electrode. This change in potential and its subsequent relaxation(s) are analyzed quantitatively in terms of: (1) A junction potential between the (hot) electrode and the (cold) contact wire; (2) a change in the potential across the electrode double layer which can be effected by a change in the capacitance and/or by a change in dipole orientation (equivalent to a change in the potential of zero charge) and/or by charge transfer (electron transfer between the electrode and a redox system located at the outer Helmholtz plane (OHP) or inner Helmholtz plane (IHP), or ion transfer between the IHP and the OHP); (3) a Soret potential. For small temperature changes and a constant charge-transfer resistance the theoretical analysis yields an analytic equation. Using a flashlamp-pumped dye laser (width at half-max. ~ 0.35 μs) and a 25 μs Pt-foil electrode, the thermal relaxation phenomena are verified and calibrated experimentally and (as a demonstration) the method is used to evaluate the heterogeneous rate constant for ferri/ferrocyanide. The possibility of probing electrode responses in the sub-microsecond and sub-nanosecond time domains is discussed.