Adsorbate-induced changes in the broadband infrared reflectance of a metal: Oxygen on Cu(100)

We have measured the adsorbate-induced change in the infrared reflectance of a Cu(100) surface upon adsorption of oxygen at room temperature, at four oxygen coverages, over the frequency range from 100 to 2000 ${\mathrm{cm}}^{\mathrm{-}1}$. The observed frequency dependence provides an independent test of a model of conduction-electron scattering from disordered adsorbates that incorporates nonlocal electrodynamics. We find good agreement with theory for coverages below 0.25 ML. The value of the characteristic roll-off frequency ${\mathrm{\omega }}_{\mathit{p}}$ ${\nu }_{\mathit{F}}$/c, however, which is predicted to be independent of the adsorbate, is different from that found for CO on the same crystal. At 0.35-ML coverage, the frequency dependence deviates strongly from the predicted form and the magnitude of the reflectance change decreases. It is possible that these deviations are related to an incipient ordering and reconstruction of the surface.