Reflectance drops and surface plasmons on different roughness deposits of magnesium

Optical measurements of reflectance of Mg deposits prepared under static ultrahigh vacuum, are presented for photon energies from 1 to 6 eV. The structure of these deposits was determined, in situ, using a replication technique. It is shown that the reflectance $R^{\prime }$ measured with photons incident through the substrate, agrees with the reflectance $R_{\mathrm{Drude}}^{\prime }$ calculated assuming a Drude model for a nearly-free-electron metal, whereas the reflectance $R$ measured with photons incident through vacuum does not agree with $R_{\mathrm{Drude}}$ and exhibits a significant decrease near the surface-plasmon energy. This decrease is explained by means of Elson and Ritchie's quantum theory on the influence of roughness on scattering and surface-plasmon excitation. Different roughness deposits of Mg are studied and their rms ${\delta }^2$ is determined by direct measurement. These results are compared with those of Gesell et al., recently published in this journal.