Roughening of Ag surfaces by Ag deposits studied by differential reflectivity

Differential reflectivity $\frac{\Delta R}{R}$ measurements between a smooth Ag surface and the same surface covered by silver deposits of various thicknesses were performed in ultrahigh vacuum over the (1.5-5)-eV spectral range for different Ag substrate temperatures. One silver monolayer on silver substrate at 125 K gives rise to relatively large changes in reflectivity (~ ${10}^{-3\mathrm{}}$). For thicker deposits (four monolayers or more) a well-defined absorption at 3.5 eV due to surface-plasmon excitation is found. The experimental results are compared to computed values of $\frac{\Delta R}{R}$. $\frac{\Delta R}{R}$ variations during sample annealing to room temperature are also investigated and compared to the reported dc resistance and surface-enhanced Raman scattering measurements. Roughness was also investigated by electron microscopy by "pinning" the surface at $T=140\mathrm{}$ K with a superficial oxide. An additional absorption found for thick (~ ${10}^3$ Å) quenched silver films is attributed to a surface effect.