Optical characterization of the evolution of ion-induced anisotropic nanopatterns on Ag(001)

Reflection anisotropy spectroscopy is used as an in situ probe for the emergence and evolution of surface patterns on Ag(001) during oblique incidence ion bombardment. The information is extracted from plasmon resonances induced by the nanoscale patterns, utilizing the fact that smooth Ag(001) is optically isotropic. The Rayleigh-Rice perturbation approach delivers the temporal development of the average periodicity and amplitude of the surface patterns. For ion bombardment at a polar angle of incidence of 70∘ along a ⟨110⟩ azimuth, strongly anisotropic surface features develop, giving rise to a single plasmon resonance, which is described well with a one-dimensional power spectral density function. For a smaller polar angle of incidence of 61.5∘ multiple plasmon resonances are observed, which demand a two-dimensional power spectral density function for a perfect description. These result compare well with high-resolution low-energy electron diffraction data, taken after ion bombardment at both angles of incidence. The optical data, obtained at 61.5∘, show coarsening and seem to suggest scaling of the periodicity and roughness, with critical exponents 0.27 and 0.40, respectively