Ripples formed in the sputter erosion of Pd(111)

During the sputter erosion of Pd(111) by 1 keV Ar⁺ ions in the temperature range ∼800-1000 K we observe the development of clearly defined ridges that deepen as sputtering continues up to ∼200 ML removed. Step edges observed by low energy electron microscopy appear as contour lines that define the detailed topography of the rippled surfaces. Different characteristic step profiles, and hence surface geometries, are observed after short and long sputtering times. The sequence resembles the interfacial structures reported for the Mullins-Sekerka instability of driven solidification interfaces. At long sputtering times the ridges adopt crystallographic orientations, as reported in recent studies of the ion erosion of crystal surfaces.