Observation of surface rippling on Cu₃u(100) with ToF-ISS

We have investigated a Cu₃Au(100) surface with ToF-ISS. The crystal was accurately oriented by use of azimuthal scans taken at grazing incidence. The polar scans measured with 2 keV Ne ions prove that the surface is unambiguously terminated by a mixed and ordered CuAu layer at room temperature. Along the 〈 100〉 azimuth, the ion beam analyses only the rows of the first monolayer, which are either pure Cu rows or pure Au rows. Along the 〈110〉 azimuth, the beam analyses rows of the first plane, which are alternated Cu-Au chains, but also the pure Cu rows in the second plane. This implies that the Cu signal is more intense in this direction than in others. The most striking result along the 〈110〉 rows is the direct measurement of rippling on the first atomic layer. We show that Au atoms lie 0.12 Å above the Cu atoms, which is in pretty good agreement with theoretical predictions.