Formation of Very Thin Oxide Films on Metals: Contact Potential Measurements during the Oxidation of (100) Cu

The coupled currents approach of Fromhold and Cook as applied to the formation of very thin oxide films on metals is modified to allow for the reflection of electrons which have tunneled to the gas–oxide interface. Reaction of the tunneling electrons with adsorbed oxygen atoms (present at low concentration) rather than with adsorbed oxygen molecules is suggested as a possible, although not exclusive, physical basis for the low capture probability. This modification leads to the hypothesis that neither a virtual electronic or ionic equilibrium may prevail across the oxide during low-temperature oxidation studies. Experimental measurements of the change in the contact potential difference between a (100) Cu single crystal and a high purity Au polycrystal during exposure to oxygen are compared with the predicted electric potential difference cross the oxide; the agreement between theory and experiment is good. A similar comparison between Rhodin's thickness measurements and predictions of the above model for the oxidation of (100) Cu shows poor quantitative agreement.