TUNNEL PROCESSES IN PHOTOELECTROCHEMICAL REACTIONS OF PASSIVE FILMS.

Previous experimental results for photocurrent measurements on passive films on iron, tantalum, and zirconium and on Xe-implanted passive films on hafnium indicate a behavior different from that of bulk cystalline materials. A discussion is presented that tries to explain cathodic photocurrents that are observed at potentials above the flatband potential and wavelength-dependent potentials where the photocurrent changes sign. The model emphasizes tunneling processes of photoexcited carriers from the passive film directly into the electrolyte. Depending on conditions such as photon energy and field, a direct competition of tunneling with and against the field which corresponds to anodic and cathodic photocurrents, respectively, is possible. Model calculations are used to support the proposed mechanism.