Cross effect of electrode absorption and electrolyte absorption in electrochemically modulated IR vibrational spectroscopy

In-situ infrared (IR) investigation of electrode surfaces is most often carried out by varying the electrode potential in order to efficiently separate the weak IR absorption of the surface from the much larger IR absorption of the bulk electrolyte. However, in some cases, especially for semiconducting electrodes, electrode potential variation may result in a large IR background signal, arising from electronic absorption due to the change of the electrode space-charge. We show here that this space-charge modulation may further introduce a cross effect making bulk-electrolyte absorption reappear in the final spectra. The calculations are carried out for a semiconducting electrode in internal reflection geometry, in a three-layer approximation (bulk semiconductor, space-charge layer, electrolyte). The cross effect is found to be present for p- as well as s-polarization of the IR beam. This effect however exactly vanishes for p-polarization and 45° incidence angle. This prediction, together with the calculated magnitude of the effect, has been verified experimentally for the case of GaAs and Si electrodes in contact with different electrolytes.