For the anomalous scattering experiments (ID01 beamline, ESRF) the polychromatic X-ray beam was monochromatized downstream from the undulator by a double crystal Si(111) monochromator with an energy resolution of DE = 1 eV yielding Dk/k B 10 -4 . The beam was cut to a size comparable to the GID experiments by the entrance slits directly in front of the sample. The reflected beam was recorded by two wide-opened slits of 4.0 9 4.0 mm 2 and 3.0 9 3.0 mm 2 . For line scanning under specular conditions, an avalanche photodiode (APD) was used as a point detector and placed directly behind the last slits reducing parasitic air scattering. The sample-to-detector distance was 28 cm. In order to reduce sample degradation (beam damage) during motor movement and in between scans, especially when applying low X-ray energies, a ''fast shutter'' was implemented in front of the entrance slits/absorber box and synchronized with the detector (Giewekemeyer and Salditt 2007) . Therefore, no automated absorbers could be used during reflectivity or offset scans; absorbers had to be calibrated in advance of data collection. A monitor, mounted directly in front of the sample, was used to control the beam intensity. To further reduce dose and avoid beam damage, the reflectivity curves and offset scans were recorded only in the vicinity of the Bragg reflections at calculated positions.